Micro Flashcards

Question 1

Q

The aseptic collection of blood cultures requires that the skin be cleansed with:
A. 2% iodine and then 70% alcohol solution
B. 70% alcohol and then 2% iodine or an iodophor
C. 70% alcohol and then 95% alcohol
D. 95% alcohol only
Microbiology/Apply knowledge of standard operating procedures/Specimen collection/1

Answer

A

B To attain asepsis of the skin, 70% alcohol followed by 2% iodine is used for obtaining
blood cultures.

Question 2

Q

When cleansing the skin with alcohol and then iodine for the collection of a blood
culture, the iodine (or iodophor) should remain intact on the skin for at least:
A. 10 seconds
B. 30 seconds
C. 60 seconds
D. 5 minutes
Microbiology/Apply knowledge of standard operating procedures/Specimen collection and
handling/1

Answer

A

C The iodine should remain on the skin for 1 minute because instant antisepsis does not
occur when cleansing the skin for a blood culture.

Question 3

Q

What is the purpose of adding 0.025% to 0.050% sodium polyanethol sulfonate (SPS) to
nutrient broth media for the collection of blood cultures?
A. It inhibits phagocytosis and complement
B. It promotes formation of a blood clot
C. It enhances growth of anaerobes
D. It functions as a preservative
Microbiology/Apply knowledge of standard operating procedures/Media/1

Answer

A

A SPS is used in most commercial blood culture products because it functions as an
anticoagulant and prevents phagocytosis and complement activation. In addition, SPS
neutralizes aminoglycoside antibiotics. Addition of SPS may inhibit some Neisseria
and Peptostreptococcus, but this can be reversed with 1.2% gelatin.

Question 4

Q

A flexible calcium alginate nasopharyngeal swab is the collection device of choice for
recovery of which organism from the nasopharynx?
A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Corynebacterium diphtheriae
D. Bacteroides fragilis
Microbiology/Apply knowledge of standard operating procedure/Specimen collection and
handling/1

Answer

A

C C. diphtheriae must be recovered from the deep layers of the pseudomembrane that
forms in the nasopharyngeal area. A flexible calcium alginate nasopharyngeal swab is
the best choice for collecting a specimen from the posterior nares and pharynx

Question 5

Q

Semisolid transport media, such as Amies, Stuart, or Cary-Blair, are suitable for the
transport of swabs for culture of most pathogens except:
A. Neisseria gonorrhoeae
B. Enterobacteriaceae
C. Campylobacter fetus
D. Streptococcus pneumoniae
Microbiology/Select methods/Reagents/Media/Specimen collection and handling/2

Answer

A

A Specimens for culture of N. gonorrhoeae are best if plated immediately or transported
in a medium containing activated charcoal to absorb inhibitory substances that hinder
their recovery from the specimens

Question 6

Q

Select the method of choice for recovery of anaerobic bacteria from a deep abscess.
A. Cotton fiber swab of the abscess area
B. Skin snip of the surface tissue
C. Needle aspirate after surface decontamination
D. Swab of the scalpel used for débridement
Microbiology/Apply knowledge of standard operating procedures/Specimen collection and
handling/2

Answer

A

C Anaerobic specimens are easily contaminated with organisms present on the skin or
mucosal surfaces when a swab is used. Needle aspiration of an abscess following
surface decontamination provides the least exposure to ambient oxygen.

Question 7

Q

Select the primary and differential media of choice for recovery of most fecal pathogens.
A. MacConkey, blood, birdseed, and Campylobacter (Campy) agars
B. Hektoen, MacConkey, MacConkey-Sorbitol, Campy blood, colistin–nalidixic acid (CNA)
agars; Selenite-F broth (SEL)
C. CNA and Christensen urea agars and thioglycollate media
D. Blood, Campy, Mueller-Hinton agars, and thioglycollate media
Microbiology/Select methods/Reagents/Media/Stool culture/2

Answer

A

B Hektoen agar selectively isolates pathogenic coliforms, especially Salmonella and
Shigella. MacConkey agar differentiates lactose fermenters from nonfermenters.
MacConkey with sorbitol isolates Escherichia coli 0157:H7. CNA agar contains
antibiotics that prohibit growth of gram-negative coliforms but not gram-positive
cocci. Campy blood agar contains the antibiotics cephalothin, trimethoprim,
vancomycin, polymyxin B, and amphotericin B to prevent growth of
Enterobacteriaceae, Pseudomonas spp., and fungi.

Question 8

Q

Select the media of choice for recovery of Vibrio cholerae from a stool specimen.
A. MacConkey agar and thioglycollate media
B. Thiosulfate–citrate–bile–sucrose (TCBS) agar and alkaline peptone water (APW) broth
C. Blood agar and SEL broth
D. CNA agar
Microbiology/Select methods/Reagents/Media/Stool culture/2

Answer

A

B TCBS agar is used to grow V. cholerae, which appear as yellow colonies as a result of
the use of both citrate and sucrose. APW is used as an enrichment broth and should be
subcultured to TCBS agar for further evaluation of Vibrio colonies.

Question 9

Q

CNA agar is used primarily for the recovery of:
A. Neisseria species
B. Enterobacteriaceae
C. Pseudomonas aeruginosa
D. Staphylococcus aureus
Microbiology/Select methods/Reagents/Media/Gram-positive cocci/2

Answer

A

D CNA agar inhibits the growth of gram-negative bacteria and is used to isolate grampositive
cocci from specimens. This medium is especially useful for stool and wound
cultures because these may contain large numbers of gram-negative rods

Question 10

Q

In the United States, most blood agar plates are prepared with 5% or 10% red blood
cells (RBCs) obtained from:
A. Sheep
B. Horses
C. Humans
D. Dogs
Microbiology/Select methods/Reagents/Media/Culture/1

Answer

A

A Sheep RBCs are used in blood agar plates because they are readily available and less
inhibitory than cells of other species. The type of hemolysis is determined by the
source of RBCs. Sheep RBCs are chosen because of the characteristically clear
hemolysis produced by β-hemolytic streptococci, Staphylococcus, and other pathogens
producing β-hemolysins. Sheep blood does not support the growth of Haemophilus
haemolyticus, eliminating the possibility of confusing it with β-hemolytic streptococci
in throat cultures.

Question 11

Q

All of the following are appropriate when attempting to isolate N. gonorrhoeae from a
genital specimen except:
A. Transport the genital swab in charcoal transport medium
B. Plate the specimen directly on modified Thayer-Martin (MTM) medium
C. Plate the specimen directly on New York City or Martin-Lewis agar
D. Culture specimens in ambient oxygen at 37°C
Microbiology/Select methods/Reagents/Media/Culture/1

Answer

A

D MTM, New York City, and Martin-Lewis agars contain blood factors needed to
support the growth of N. gonorrhoeae as well as antibiotics that prevent growth of
normal genital flora. Cultures must be incubated in 3% to 7% carbon dioxide (CO2) at
35°C. Cultures should be held a minimum of 48 hours before being considered
negative.

Question 12

Q

Chocolate agar and MTM agar are used for the recovery of:
A. Haemophilus spp. and Neisseria spp., respectively
B. Haemophilus spp. and N. gonorrhoeae, respectively
C. Neisseria spp. and Streptococcus spp., respectively
D. Streptococcus spp. and Staphylococcus spp., respectively
Microbiology/Select methods/Reagents/Media/Stool culture/2

Answer

A

B Chocolate agar provides X factor (hemin) and V factor (nicotinamide adenine
dinucleotide [NAD]) required for the growth of Haemophilus spp. Thayer-Martin
medium is a chocolate agar containing the antibiotics that permit isolation of N.
gonorrhoeae in specimens containing large numbers of gram-negative bacteria,
including commensal Neisseria spp.

Question 13

Q

Cycloserine–cefoxitin–fructose agar (CCFA) is used for the recovery of:
A. Yersinia enterocolitica
B. Yersinia intermedia
C. Clostridium perfringens
D. Clostridium difficile
Microbiology/Select methods/Reagents/Media/Stool culture/1

Answer

A

D CCFA is used for recovery of C. difficile from stool cultures. Cycloserine and
cefoxitin inhibit growth of gram-negative coliforms in the stool specimen. C. difficile
ferments fructose, forming acid that, in the presence of neutral red, causes the colonies
to become yellow

Question 14

Q

Deoxycholate agar (DCA) is useful for the isolation of:
A. Enterobacteriaceae
B. Enterococcus spp.
C. Staphylococcus spp.
D. Neisseria spp.
Microbiology/Select methods/Reagents/Media/Stool culture/1

Answer

A

A DCA inhibits gram-positive organisms. N. gonorrhoeae and Neisseria meningitidis
are too fastidious to grow on DCA. Citrate and deoxycholate salts inhibit growth of
gram-positive bacteria. The media contain lactose and neutral red, allowing
differentiation of lactose fermenters (pink colonies) from nonfermenters (colorless

Question 15

Q

Xylose lysine deoxycholate (XLD) agar is a highly selective medium used for the
recovery of which bacteria?
A. Staphylococcus spp. from normal flora
B. Yersinia spp. that do not grow on Hektoen agar
C. Enterobacteriaceae from gastrointestinal specimens
D. Streptococcus spp. from stool cultures
Microbiology/Select methods/Reagents/Media/Stool culture/1

Answer

A

C XLD agar is selective for gram-negative coliforms because of a high concentration (0.25%) of deoxycholate, which inhibits gram-positive bacteria. In addition, XLD is
differential for Shigella and Salmonella spp. The medium contains xylose, lactose, and
sucrose, which are fermented by most normal intestinal coliforms producing yellow
colonies. Shigella does not ferment the sugars and produces red (or clear) colonies.
Salmonella spp. ferment xylose; however, they also decarboxylate lysine in the
medium, causing production of ammonia. Therefore, Salmonella first appear yellow
but become red. Some Salmonella produce hydrogen sulfide (H2S) from sodium
thiosulfate and therefore appear as red colonies with black centers.

Question 16

Q

A sheep blood agar plate is used as a primary isolation medium when all of the
following organisms are to be recovered from a wound specimen except:
A. β-Hemolytic streptococci and coagulase-positive staphylococci
B. Haemophilus influenzae and Haemophilus parainfluenzae
C. Proteus spp. and Escherichia coli
D. Pseudomonas spp. and Acinetobacter spp.
Microbiology/Select methods/Reagents/Media/Wound culture/2

Answer

A

B Both gram-positive cocci and gram-negative bacilli will grow on blood agar plates,
but the medium is used in conjunction with a selective medium, such as CNA agar, for
gram-positive cocci and MacConkey agar for gram-negative bacilli. H. influenzae
requires X and V factors, and H. parainfluenzae requires V factor; the primary
isolation medium for Haemophilus is chocolate agar.

Question 17

Q

Prereduced and vitamin K1-supplemented blood agar plates are recommended isolation
media for:
A. Mycobacterium marinum and Mycobacterium avium intracellulare
B. Bacteroides, Peptostreptococcus, and Clostridium spp.
C. Proteus spp.
D. Enterococcus spp.
Microbiology/Select methods/Reagents/Media/Anaerobes/2

Answer

A

B Anaerobic culture media can be prereduced before sterilization by boiling, saturation
with oxygen-free gas, and addition of cysteine or other thiol compounds. The final
oxidation reduction potential (Eh) of the medium should be approximately –150 mV to
minimize the effects of exposure of organisms to oxygen during inoculation.

Question 18

Q

Which procedure(s) is (are) appropriate for the diagnosis of Chlamydia spp. infections
when using genital specimens?
A. Obtain urethral, cervical swabs and urine specimens placed in transport media for the
direct detection of antigen or nucleic acid and/or culture
B. Plate onto blood and chocolate agar
C. Inoculate into thioglycollate (THIO) broth
D. Plate onto MTM agar within 24 hours
Microbiology/Select methods/Reagents/Media/Virus culture/1

Answer

A

A Chlamydiae are strict intracellular organisms and must be cultured using living cells
(e.g., cyclohexamide-treated McCoy cells). Direct smears can also be made at the time
of culture. Fluorescein-conjugated monoclonal antibodies may be used to identify the
organisms in infected cells. Cell cultures present limitations but are used if legal
situations (e.g., sexual abuse) are implied. Commercially available kits for antigen
detection and nucleic acid amplification and hybridization techniques, which have
been approved by the U.S. Food and Drug Administration (FDA), are not culture
methods but appear more reliable for the detection of infection in individuals who are
symptomatic and shedding large numbers of organisms

Question 19

Q

Specimens for virus culture should be transported in media containing:
A. Antibiotics and 5% sheep blood
B. Saline and 5% sheep blood
C. 22% bovine albumin
D. Antibiotics and protein nutrient
Microbiology/Select methods/Reagents/Media/Virus culture/1

Answer

A

D Media for transporting specimens for virus culture include Hanks balanced salt
solution with bovine albumin, Amies media, Stuart transport media, and Leibovitz-
Emory media. Media used for transporting specimens for viral culture are similar to
those for bacteria with the addition of a nutrient, such as fetal calf serum or albumin,
and antibiotics. Specimens should be refrigerated (not frozen) after being placed in the
transport media until the culture media can be inoculated

Question 20

Q

Cerebrospinal fluid (CSF) should be cultured immediately, but if delayed, the specimen
should be:
A. Refrigerated at 4°C to 6°C
B. Frozen at –20°C
C. Stored at room temperature for no longer than 24 hours
D. Incubated at 37°C and cultured as soon as possible
Microbiology/Apply knowledge of standard operating procedures/Specimen collection and
transport/1

Answer

A

D Fastidious organisms, such as Neisseria and Haemophilus, frequently isolated from
the CSF of patients with bacterial meningitis are preserved by placing the fluid in 3%
to 7% CO2 at 35°C to 37°C (or at room temperature for no longer than 30 minutes), if
the specimen cannot be cultured immediately

Question 21

Q

The most sensitive method for the detection of β-lactamase in bacteria is by the use of:
A. Chromogenic cephalosporin
B. Penicillin
C. Oxidase
D. Chloramphenicol acetyltransferase
Microbiology/Select methods/Reagents/Media/Sensitivity testing/2

Answer

A

A β-Lactamase production by bacteria that are resistant to penicillin and cephalosporin
is detected using one of these drugs as a substrate. Penicillin is hydrolyzed by β-
lactamase into acidic products that can be detected as a color change by a pH indicator.
In the iodometric method, a disk containing a penicillin–starch substrate turns blue
when a drop of iodine is added. The most sensitive method of detection is based on the
ability of the organism to hydrolyze the β-lactam ring of a chromogenic cephalosporin
(e.g., nitrocefin), which is used as the substrate. A positive test indicates the
organism’s resistance to penicillin, ampicillin, amoxicillin, carbenicillin, mezlocillin,
and piperacillin.

Question 22

Q

The breakpoint of an antimicrobial drug refers to:
A. The amount needed to cause bacteriostasis
B. A minimum inhibitory concentration (MIC) of 16 μg/mL or greater
C. An MIC of 64 μg/mL or greater
D. The optimal therapeutic level of drug that is achievable in serum
Microbiology/Apply principle of theory and practice related to laboratory
operations/Sensitivity testing/2

Answer

A

D The term breakpoint refers to an antimicrobial concentration in the serum associated
with optimal therapy using the customary dosing schedule. An organism is susceptible
if the MIC is at or below the breakpoint

Question 23

Q

Which of the following variables may change the results of an MIC?
A. Inoculum size
B. Incubation time
C. Growth rate of the bacteria
D. All of these options
Microbiology/Apply knowledge to identify sources of error/Sensitivity testing/2

Answer

A

D In vitro testing of drugs is reliable if the method is standardized. In addition to the
first three variables, the type of media and the stability of antibiotics affect the results
of MIC testing and must be carefully controlled

Question 24

Q

According to the Kirby-Bauer standard antimicrobial susceptibility testing method,
what should be done when interpreting the zone size of a motile, swarming organism,
such as a Proteus species?
A. The swarming area should be ignored
B. The results of the disk diffusion method are invalid
C. The swarming area should be measured as the growth boundary
D. The isolate should be retested after diluting to a 0.05 McFarland standard
Microbiology/Apply knowledge of standard operating procedures/Sensitivity testing/2

Answer

A

A A thin film of growth appearing in the zone area of inhibition around the
susceptibility disk should be ignored when swarming Proteus or other organisms are
encountered. Discontinuous, poor growth or tiny colonies near the end of the zone
should also be ignored.

Question 25

Q

Which class of antibiotics is used for the treatment of serious gram-negative infections
as well as infections with Mycobacterium tuberculosis?
A. Cephalosporins
B. Penicillins
C. Tetracyclines
D. Aminoglycosides
Microbiology/Apply knowledge of fundamental biological characteristics/Antibiotics/1

Answer

A

D The aminoglycoside antibiotics are bactericidal agents that act by inhibiting protein synthesis. They show a low incidence of bacterial resistance but must be monitored
carefully because at high doses they can cause ototoxicity and nephrotoxicity. The
group includes amikacin, gentamicin, tobramycin, kanamycin, streptomycin, and
spectinomycin. These drugs are usually administered intravenously or intramuscularly
because they are poorly absorbed from the gastrointestinal tract

Question 26

Q

Select the medium best suited for the recovery of Y. enterocolitica from a patient with
gastroenteritis.
A. Hektoen agar
B. Cefsulodin–irgasan–novobiocin (CIN) agar
C. Blood agar
D. Eosin methylene blue agar
Microbiology/Select methods/Reagents/Media/Gram-negative bacilli/2

Answer

A

B CIN agar inhibits the growth of many organisms from the family Enterobacteriaceae.
Yersinia spp. are also recovered from MacConkey and Salmonella-Shigella agars. Y.
enterocolitica are seen as “bull’s eye” colonies on CIN agar after 48 hours at room
temperature.

Question 27

Q

A suspected case of plague requires which of the following procedures to confirm
Yersinia pestis?
A. Collection of multiple sets of blood culture specimens
B. Incubation of blood cultures at both 28°C and 35°C
C. Culture aspirates from bubos to MacConkey agar at room temperature
D. All of these options
Microbiology/Apply knowledge of procedures/Gram-negative bacilli/2

Answer

A

D Y. pestis is on the list of agents of bioterrorism. Isolation and identification should be
performed in a facility with a Level II or higher biosafety rating. If there is a high risk
of aerosolizing the specimen during processing, procedures should be performed under
Level III biosafety conditions. Recovery of Y. pestis is highest if the specimen is
cultured within 2 hours of collection

Question 28

Q

SITUATION: Abdominal pain, fever, vomiting, and nausea prompted an older male to
seek medical attention. A watery stool specimen producing no fecal leukocytes or
erythrocytes was cultured, and it grew a predominance of gram-negative fermentative
bacilli. The colonies were β-hemolytic on blood agar and cream colored on MacConkey
agar. The colonies were both oxidase and catalase positive. What is the most likely
identification?
A. Aeromonas hydrophilia
B. Escherichia coli
C. Salmonella spp.
D. Shigella spp.
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/3

Answer

A

A The oxidase positive test result rules out the members of the Enterobacteriaceae
family. Colonies of A. hydrophilia and Plesiomonas spp. (both oxidase positive) might
be mistaken for Vibrio spp. because all three grow as clear colonies on MacConkey
agar, are β-hemolytic on blood agar, and are oxidase positive. Aeromonas spp. will
also grow on CNN agar and mimic Y. enterocolitica (a member of the
Enterobacteriaceae family and oxidase negative).

Question 29

Q

SITUATION: Several attendees of a medical conference in the Gulf coast area became
ill after frequenting a seafood restaurant. A presumptive identification of V. cholerae
was made after stool specimens from several subjects grew clear colonies on
MacConkey agar and yellow colonies on TCBS agar. Which key tests would help
eliminate Aeromonas and Plesiomonas spp.?
A. Mannitol fermentation, Na+ requirement
B. Oxidase, motility
C. Oxidase, nitrate
D. Hemolysis on blood agar, catalase
Microbiology/Select methods/Reagents/Media/Gram-negative bacilli/3

Answer

A

A All three organisms are positive for oxidase production and are motile. Plesiomonas
spp. do not grow on TCBS agar. Clear colonies on MacConkey agar and yellow
colonies on TCBS agar indicate Vibrio or Aeromonas spp. However, only Vibrio spp.
require Na+ (1% NaCl) in the medium for growth.

                   Vibrio                  Aeromonas                       Plesiomonas Oxidase            +                             +                                           + Na+ requirement +                     Neg                                      Neg Mannitol fermentation +            +                                          Neg Growth onTCBS +                          +                                         Neg

Question 30

Q

SITUATION: A group of elementary students became ill after eating undercooked
ground beef prepared in the school cafeteria. The suspected pathogen, E. coli serotype
0157:H7, is usually recovered using which of the following media?
A. Xylose lysine deoxycholate agar
B. MacConkey agar
C. MacConkey agar with sorbitol
D. Hektoen agar
Microbiology/Select methods/Reagents/Media/Gram-negative bacilli/2

Answer

A

C E. coli 0157:H7 ferments lactose and, therefore, appears as dark pink colonies on
MacConkey agar. To differentiate E. coli 0157:H7 from normal fecal flora,
MacConkey agar with sorbitol is used. E. coli 0157:H7 does not ferment sorbitol, and
usually are colorless colonies.

Question 31

Q

Biochemically, Enterobacteriaceae species are gram-negative rods that:
A. Ferment glucose, reduce nitrate to nitrite, and are oxidase negative
B. Ferment glucose, produce indophenol oxidase, and form gas
C. Ferment lactose and reduce nitrite to nitrogen gas
D. Ferment lactose and produce indophenol oxidase
Microbiology/Apply knowledge of fundamental biolog

Answer

A

A The family Enterobacteriaceae consists of more than 100 species and represents the
most commonly encountered isolates in clinical specimens. All Enterobacteriaceae
species ferment glucose, are nonsporulating, and are oxidase negative (except for
Plesiomonas shigelloides, recently added to the family but which is oxidase positive).
Most Enterobacteriaceae species are motile, but the genera Shigella and Klebsiella are
not.

Question 32

Q

The ortho-nitrophenyl-β-galactopyranoside (ONPG) test is most useful when
differentiating:
A. Salmonella spp. from Pseudomonas spp.
B. Shigella spp. from some strains of Escherichia coli
C. Klebsiella spp. from Enterobacter spp.
D. Proteus vulgaris from Salmonella spp.
Microbiology/Apply principles of basic laboratory procedures/Biochemical/2

Answer

A

B The ONPG test detects β-galactosidase activity and is most useful in distinguishing late
lactose fermenters from lactose nonfermenters. Some strains of E. coli are slow lactose
fermenters and may be confused with Shigella spp., which do not ferment lactose. E. coli are ONPG positive, whereas Shigella spp. are ONPG negative.

Question 33

Q

The Voges-Proskauer (VP) test detects which end product of glucose fermentation?
A. Acetoin
B. Nitrite
C. Acetic acid
D. Hydrogen sulfide
Microbiology/Apply principles of basic laboratory procedures/Biochemical/1

Answer

A

A Acetoin or carbinol, an end product of glucose fermentation, is converted to diacetyl
after the addition of the VP reagents (β-naphthol and 40% potassium hydroxide
[KOH]). Diacetyl is seen as a red- to-pink–colored complex.

Question 34

Q

At which pH does the methyl red (MR) test become positive?
A. 7.0
B. 6.5
C. 6.0
D. 4.5
Microbiology/Apply principles of basic laboratory procedures/Biochemical/1

Answer

A

D Both MR and VP tests detect acid production from the fermentation of glucose.
However, a positive MR test result denotes a more complete catabolism of glucose to
highly acidic end products, such as formate and acetate, than occurs with organisms
that are VP positive only (e.g., Klebsiella pneumoniae).

Question 35

Q

A positive Simmons citrate test is seen as a:
A. Blue color in the medium after 24 hours of incubation at 35°C
B. Red color in the medium after 18 hours of incubation at 35°C
C. Yellow color in the medium after 24 hours of incubation at 35°C
D. Green color in the medium after 18 hours of incubation at 35°C
Microbiology/Apply principles of basic laboratory procedures/Biochemical/1

Answer

A

A The Simmons citrate test determines if an organism can utilize citrate as the sole source
of carbon. The medium turns blue, indicating the presence of alkaline products, such as
carbonate. Tubes are incubated a minimum of 24 hours at 35°C with a loose cap before
reading

Question 36

Q

In the test for urease production, ammonia reacts to form which product?
A. Ammonium citrate
B. Ammonium carbonate
C. Ammonium oxalate
D. Ammonium nitrate
Microbiology/Apply principles of basic laboratory procedures/Biochemical/1

Answer

A

B The test for urease production is based on the ability of the colonies to hydrolyze urea
in Stuart broth or Christensen agar to form CO2 and ammonia. These products form
ammonium carbonate, resulting in alkalinization. This turns the pH indicator (phenol
red) pink at pH 8.0

Question 37

Q

Which of the following reagents is added to detect the production of indole?
A. p-Dimethylaminobenzaldehyde
B. Bromcresol purple
C. Methyl red
D. Cytochrome oxidase
Microbiology/Apply principles of basic laboratory procedures/Biochemical/1

Answer

A

A The indole test detects the conversion of tryptophan (present in the media) to indole by
the enzyme tryptophanase. Indole is detected by the reaction with the aldehyde group
of p-dimethylaminobenzaldehyde (the active reagent in Kovac and Ehrlich reagents) in
acid, forming a red complex

Question 38

Q

Decarboxylation of the amino acids lysine, ornithine, and arginine results in the
formation of:
A. Ammonia
B. Urea
C. Carbonate
D. Amines
Microbiology/Apply principles of basic laboratory procedures/Biochemical/1

Answer

A

D Specific decarboxylases split dibasic amino acids (lysine, arginine, and ornithine),
forming alkaline amines. These products turn the pH indicators in the medium (cresol
red and bromcresol purple) from yellow to purple.

Question 39

Q

Lysine iron agar (LIA) showing a purple slant and a blackened butt indicates:
A. Escherichia coli
B. Citrobacter spp.
C. Salmonella spp.
D. Proteus spp.
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/2

Answer

A

C LIA is used as an aid for the identification of Salmonella species. It contains
phenylalanine, lysine, glucose, thiosulfate, ferric ammonium citrate, and bromcresol
purple. Salmonella spp. produce H2S from thiosulfate. This reduces ferric ammonium
citrate, forming ferrous sulfate and causing the butt to blacken. Salmonella also
decarboxylate lysine to produce alkaline amines, giving the slant its purple color and
differentiating it from Citrobacter spp., which are lysine decarboxylase negative

Question 40

Q

Putrescine is an alkaline amine product of which bacterial enzyme?
A. Arginine decarboxylase
B. Phenylalanine deaminase
C. Ornithine decarboxylase
D. Lysine decarboxylase

Answer

A

C Putrescine is the amine product of the decarboxylation of ornithine.

Question 41

Q

Which genera are positive for phenylalanine deaminase?
A. Enterobacter, Escherichia, and Salmonella
B. Morganella, Providencia, and Proteus
C. Klebsiella and Enterobacter
D. Proteus, Escherichia, and Shigella
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/2

Answer

A

B Phenylalanine deaminase oxidatively deaminates phenylalanine, forming
phenylpyruvic acid. When a solution of ferric chloride is added, the iron reacts with
phenylpyruvic acid, forming a green-colored complex. Phenylalanine deaminase is
found in the genera Morganella, Providencia, and Proteus and is an excellent test to
determine if an organism belongs to this group. Rarely, isolates of Enterobacter may
be phenylalanine deaminase positive as well.

Question 42

Q

Kligler iron agar (KIA) differs from triple-sugar iron agar (TSI) in the:
A. Ratio of lactose to glucose
B. Ability to detect H2S production
C. Use of sucrose in the medium
D. Color reaction denoting production of acid
Microbiology/Apply principles of basic laboratory
procedures/Methods/Reagents/Media/Gram-negative bacilli/2

Answer

A

C Both KIA and TSI contain 10-fold more lactose than glucose, peptone, and phenol red
to detect acid production (turns yellow) and sodium thiosulfate and ferrous ammonium
sulfate to detect H2S production. However, TSI contains sucrose, and KIA does not.
Organisms fermenting either sucrose or lactose will turn the slant of the agar tube
yellow. Therefore, some organisms (e.g., many species of Cedecea, Citrobacter,
Edwardsiella, and Serratia) will produce a yellow slant on TSI but a red slant on KIA.

Question 43

Q

The malonate test is most useful in differentiating which members of the
Enterobacteriaceae family?
A. Shigella
B. Proteus
C. Salmonella subgroups 2, 3 (the former Arizona)
D. Serratia
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/2

Answer

A

C The malonate test determines whether an organism can utilize sodium malonate as the
sole source of carbon. Malonate is broken down, forming alkaline metabolites that
raise the pH of the broth above 7.6. This causes bromthymol blue to turn from green to
deep blue (Prussian blue). E. coli, Shigella, and most Salmonella are malonate
negative, whereas Enterobacter and Salmonella (formerly Arizona) subgroups 2, 3a,
and 3b are positive. Proteus, Providencia, Serratia, and Yersinia are also malonate
negative.

Question 44

Q

Which genera of the Enterobacteriaceae family are known to cause diarrhea and are
considered enteric pathogens?
A. Enterobacter, Klebsiella, Providencia, and Proteus
B. Escherichia, Salmonella, Shigella, and Yersinia
C. Pseudomonas, Moraxella, Acinetobacter, and Aeromonas
D. Enterobacter, Citrobacter, and Morganella
Microbiology/Apply knowledge of fundamental biological characteristics/Gram-negative
bacilli/1

Answer

A

B Escherichia, Salmonella, Shigella, and Yersinia are responsible for the majority of
enteric diarrhea cases attributable to the Enterobacteriaceae family.

Question 45

Q

An isolate of E. coli recovered from the stool of a patient with severe bloody diarrhea
should be tested for which sugar before sending it to a reference laboratory for
serotyping?
A. Sorbitol (fermentation)
B. Mannitol (oxidation)
C. Raffinose (fermentation)
D. Sucrose (fermentation)
Microbiology/Evaluate laboratory data to recognize health and disease states/Gramnegative
bacilli/3

Answer

A

A An isolate of E. coli (shiga-like producing toxin of E. coli [STEC]) recovered from a
stool culture in hemorrhagic colitis can be definitely identified only by serotyping. The
isolate is identified as E. coli by the usual biochemical reactions. The strain of E. coli
responsible for hemorrhagic colitis is O157:H7 and is usually negative for sorbitol
fermentation. Colonies of this strain of E. coli appear colorless on MacConkey agar
with sorbitol added.

Question 46

Q

Care must be taken when identifying biochemical isolates of Shigella because
serological cross reactions occur with:
A. Escherichia coli
B. Salmonella spp.
C. Pseudomonas spp.
D. Proteus spp.
Microbiology/Apply knowledge of fundamental biological characteristics/Gram-negative
bacilli/2

Answer

A

A Serological confirmation of Shigella isolates is based on O antigen typing. If a
suspected Shigella spp. is serologically typed with polyvalent sera before it has been
correctly identified biochemically, a false-positive confirmation may occur with an
isolate that is E. coli (i.e., anaerogenic, non–gas-producing, lactose-negative or -
delayed, and nonmotile strains). These strains were formerly known as the
Alkalescens-Dispar serotype.

Question 47

Q

Which species of Shigella is most commonly associated with diarrheal disease in the
United States?
A. Shigella dysenteriae
B. Shigella flexneri
C. Shigella boydii
D. Shigella sonnei
Microbiology/Apply knowledge of fundamental biological characteristics/Gram-negative
bacilli/2

Answer

A

D The Shigella spp. are lactose nonfermenters that, for the most part, are biochemically
inert and are classified into serogroups A, B, C, and D as a result of their biochemical
similarity. S. sonnei is the species most often isolated from diarrhea cases in the United
States. It is more active biochemically than the other species because of ornithine
decarboxylase and β-galactosidase activity. These enzymes, found in most strains of S.
sonnei, distinguish it from other Shigella species.

Question 48

Q

Which of the following tests best differentiates Shigella species from E. coli?
A. H2S, VP, citrate, and urease
B. Lactose, indole, ONPG, and motility
C. Hydrogen sulfide, MR, citrate, and urease
D. Gas, citrate, and VP
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/2

Answer

A

B E. coli, when positive for lactose, indole, and ONPG, are usually motile. Shigella
species do not ferment lactose or produce indole, lack β-galactosidase, and are
nonmotile.

Question 49

Q

Which genera of Enterobacteriaceae are usually nonmotile at 36°C?
A. Shigella, Klebsiella, and Yersinia
B. Escherichia, Edwardsiella, and Enterobacter
C. Proteus, Providencia, and Salmonella
D. Serratia, Morganella, and Hafnia
Microbiology/Apply knowledge of fundamental biological characteristics/Gram-negative
bacilli/2

Answer

A

A Shigella spp. and Klebsiella spp. are, for the most part, nonmotile. Yersinia can be
motile at 22°C but is nonmotile at 36°C. Other members of Enterobacteriaceae that
have been isolated from human specimens and are usually nonmotile include
Leminorella, Rahnella, and Tatumella (considered unusual enteric pathogens).

Question 50

Q

Cold enrichment of feces (incubation at 4°C) in phosphate-buffered saline prior to
subculture onto enteric media enhances the recovery of:
A. Enterotoxigenic Escherichia coli
B. Salmonella paratyphi
C. Hafnia alvei
D. Yersinia enterocolitica
Microbiology/Apply principles of special procedures/Gram-negative bacilli/2

Answer

A

D Cold enrichment is especially useful when specimens contain large numbers of
normal flora that are sensitive to prolonged exposure to near-freezing temperature. In
addition to Yersinia, the technique has been used to enhance recovery of Listeria
monocytogenes from specimens containing other bacteria

Question 51

Q

Fever, abdominal cramping, watery stools, and fluid and electrolyte loss preceded by bloody stools 2 to 3 days before is characteristic of shigellosis but may also result from
infection with:
A. Campylobacter spp.
B. Salmonella spp.
C. Proteus spp.
D. Yersinia spp.
Microbiology/Apply knowledge of fundamental biological characteristics/Gram-negative
bacilli/2

Answer

A

A Shigella spp. and Campylobacter spp. are both causes of diarrhea, abdominal pain,
fever, and sometimes vomiting. Blood is present in the stools of patients infected with
Shigella as a result of invasion and penetration of the bowel by the organisms. Young
children may also exhibit bloody stools when infected with Campylobacter.

Question 52

Q

A routine, complete stool culture procedure should include media for the isolation of E.
coli O157:H7 as well as:
A. Salmonella, Shigella, Yersinia, Campylobacter, and Staphylococcus aureus
B. Vibrio cholerae, Brucella, and Yersinia spp.
C. Staphylococcus aureus, group B streptococci, and group D streptococci
D. Clostridium difficile, Clostridium perfringens, and Yersinia spp.
Microbiology/Select methods/Reagents/Media/Gram-negative bacilli/2

Answer

A

A V. cholerae and C. difficile are usually not included in a routine stool culture. If
Vibrio spp. are suspected, a special request should be included. Although MacConkey agar will support the growth of Vibrio spp., normal enteric flora overgrow and occlude
these organisms. C. difficile culture requires special media (e.g., CCFA) that inhibit
other anaerobic flora and facultative anaerobic flora, and culture should be requested
specifically if symptoms warrant. MacConkey agar with sorbitol will allow the E. coli
O157:H7 to be recovered. Yersinia spp. can be detected on a regular MacConkey agar
plate.

Question 53

Q

Which group of tests, along with colonial morphology on primary media, aids most in
the rapid identification of Enterobacteriaceae?
A. MR and VP, urease, and blood agar plate
B. Phenylalanine deaminase, urease, and CDC agar plate
C. Bacitracin, β-lactamase, and MacConkey agar plate
D. Indole, oxidase, MacConkey, and blood agar plates
Microbiology/Select methods/Reagents/Media/Gram-negative bacilli/2

Answer

A

D All Enterobacteriaceae species are oxidase negative (the exception being Plesiomonas
spp., which cluster with the genus Proteus). Through the use of nucleic acid–based
methods, oxidase positive Plesiomonas is now included in the family
Enterobacteriaceae. Because E. coli and Proteus spp. comprise the majority of the
organisms recovered from clinical specimens, they can be initially identified through
rapid testing without additional overnight testing. E. coli display a positive indole test,
and the colonial morphology on MacConkey agar is distinctive, showing flat, pink
(lactose-positive) colonies with a ring of bile precipitation. Proteus spp. swarm on
blood agar and are indole negative.

Question 54

Q

Which group of tests best identifies the Morganella and Proteus genera?
A. Motility, urease, and phenylalanine deaminase
B. Malonate, glucose fermentation, and deoxyribonuclease (DNase)
C. Indole, oxidase, MR, and VP
D. Indole, citrate, and urease
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/2

Answer

A

A Morganella and Proteus spp. are motile, produce urease, and deaminate
phenylalanine

Question 55

Q

Which group of tests best differentiates Enterobacter aerogenes from Edwardsiella
tarda?
A. Motility, citrate, and urease
B. H2S production, sucrose fermentation, indole, and VP
C. Lysine decarboxylase, urease, and arginine dihydrolase
D. Motility, H2S production, and DNase
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/2

Answer

A

B
Test E. aerogenes (% positive) E. tarda (% positive)
H2S 0 100
Sucrose >90 0
Indole <20 100
VP 100 0
Citrate 95 0

Question 56

Q

Cronobacter sakazakii (formerly Enterobacter sakazakii) can best be differentiated from
Enterobacter cloacae by which of the following characteristics?
A. Yellow pigmentation and negative sorbitol fermentation
B. Pink pigmentation and positive arginine dihydrolase
C. Yellow pigmentation and positive urease
D. H2S production on TSI
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/2

Answer

A

A C. sakazakii was formally called a yellow-pigmented E. cloacae and is best
differentiated from E. cloacae by sorbitol fermentation (95% positive for E. cloacae
and 0% for [E.] C. sakazakii). In addition, E. cloacae is usually positive for urease and
malonate (65% and 75%, respectively) and (E.) C. sakazakii is usually negative (1%
and less than 20%, respectively). Both are usually motile and arginine dihydrolase
positive.

Question 57

Q

Members of the genus Cedecea are best differentiated from Serratia spp. by which test
result?
A. Positive motility
B. Positive urease
C. Positive phenylalanine deaminase
D. Negative DNase
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/2

Answer

A

D DNase is not produced by Cedecea spp. but is produced (along with proteinases) by
Serratia spp. Other key differential tests include lipase (positive for Cedecea, negative
for Serratia) and gelatin hydrolysis (negative for Cedecea, positive for Serratia).

Question 58

Q

Which of the following organisms is often confused with the Salmonella species
biochemically and on plated media?
A. Escherichia coli
B. Citrobacter freundii
C. Enterobacter cloacae
D. Shigella dysenteriae
Microbiology/Apply knowledge of fundamental biological characteristics/Gram-negative
bacilli/2

Answer

A

B Biochemical differentiation is essential because Citrobacter isolates may give a falsepositive
result on agglutination testing with Salmonella grouping sera. C. freundii
strains, like Salmonella spp., are usually H2S producers and may be confused with
Salmonella spp. unless the proper biochemical tests are utilized. C. freundii and
Salmonella spp. are adonitol, indole, and malonate negative. However, C. freundii is
KCN positive, whereas Salmonella spp. are KCN negative. (C. freundii are late lactose
fermenters and appear colorless at 24 hours on Mac).

Question 59

Q

A gram-negative rod is recovered from a catheterized urine sample from a nursing
home patient. The lactose-negative isolate tested positive for indole, urease, ornithine
decarboxylase, and phenylalanine deaminase and negative for H2S. The most probable
identification is:
A. Edwardsiella spp.
B. Morganella spp.
C. Ewingella spp.
D. Shigella spp.
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/3

Answer

A

B Morganella spp. are biochemically similar to Proteus spp., both being lactose
negative, motile, and positive for phenylalanine deaminase and urease. However,
Morganella spp. can be differentiated from Proteus spp. based on H2S, indole,
ornithine decarboxylase, and xylose fermentation. Ewingella spp. are usually positive
(70%) for lactose fermentation, whereas the other three genera are lactose negative

Question 60

Q

Which single test best separates Klebsiella oxytoca from K. pneumoniae?
A. Urease
B. Sucrose
C. Citrate
D. Indole
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/2

Answer

A

D K. oxytoca and K. pneumoniae are almost identical biochemically except for the
ability to produce indole. Both organisms are nonmotile and usually test positive for
urease, sucrose, and citrate. However, K. oxytoca is indole positive, and K.
pneumoniae is indole negative.

Question 61

Q

Which of the following organisms, found in normal fecal flora, may be mistaken
biochemically for the genus Yersinia?
A. Klebsiella spp.
B. Proteus spp.
C. Escherichia coli
D. Enterobacter spp.
Microbiology/Apply knowledge of fundamental biological characteristics/Gram-negative
bacilli/2

Answer

A

B Proteus spp. are urease positive as are approximately 70% of Y. enterocolitica
isolates. Both organisms are lactose negative and motile. However, Yersinia is motile
at 22°C and not at 35°C (demonstrated by using motility media).

Question 62

Q

Why might it be necessary for both pink (lactose-positive) and colorless (lactosenegative)
colonies from an initial stool culture on MacConkey agar to be subcultured
and tested further for possible pathogens?
A. Most Shigella strains are lactose positive
B. Most Salmonella strains are maltose negative
C. Most Proteus spp. are lactose negative
D. Pathogenic Escherichia coli can be lactose positive or lactose negative
Microbiology/Evaluate laboratory data to make identifications/Gram-negative bacilli/2

Answer

A

D Possible pathogenic strains of E. coli should be picked from MacConkey agar and
subcultured onto MacConkey agar with sorbitol. After subculture, these strains can be
serotyped or sent to a reference laboratory. Most E. coli normal flora ferment Dsorbitol
and appear pink to red on MacConkey–sorbitol agar. The E. coli strain
O157:H7 causes the enteric disease hemorrhagic colitis. It ferments D-sorbitol slowly
or not at all and appears as colorless colonies on MacConkey–sorbitol agar.

Question 63

Q

Which agar that is used for routine stool cultures is the medium of choice for the
isolation of Yersinia strains from stool specimens?
A. Salmonella–Shigella agar
B. Hektoen enteric agar
C. MacConkey agar
D. CNA agar
Microbiology/Select methods/Reagents/Media/Gram-negative bacilli/2

Answer

A

C CIN medium is the best agar for the isolation of Yersinia strains because it inhibits
growth of other coliforms, but it is not used routinely in clinical laboratories. Yersinia
spp. grow well on MacConkey agar incubated at 37°C, but the colonies are much
smaller than the other Enterobacteriaceae species; therefore, 25°C is the temperature
recommended for isolation. Some serotypes of Yersinia may be inhibited on more
selective media, such as Salmonella–Shigella or Hektoen. CNA agar inhibits the
growth of gram-negative bacteria.

Question 64

Q

Which organism is sometimes mistaken for Salmonella and will agglutinate in
Salmonella polyvalent antiserum?
A. Citrobacter freundii strains
B. Proteus mirabilis strains
C. Shigella sonnei strains
D. Escherichia coli
Microbiology/Apply knowledge of fundamental biological characteristics/Gram-negative
bacilli/2

Answer

A

A C. freundii and Salmonella spp. are H2S positive and indole, VP, and phenylalanine
deaminase negative. Biochemical characteristics that help to differentiate C. freundii
from Salmonella include lactose fermentation (50% of C. freundii are lactose positive,
whereas 100% of Salmonella are lactose negative) and urease production (70% of
Citrobacter are positive and greater than 99% of Salmonella are negative).

Answer 65

A

A Shigella is the most likely organism biochemically. E. coli are usually indole and
motility positive, and Proteus are motility and urease positive. Most Salmonella are
H2S positive. Shigella and Campylobacter cause bloody diarrhea because they invade
the epithelial cells of the large bowel; however, Campylobacter spp. do not grow on
MacConkey agar, and they are oxidase positive.

Answer 66

A

D Enterobacteriaceae species are grouped according to their ability to ferment lactose, a
β-galactoside. Salmonella, Shigella, Proteus, Providencia, and Morganella are usually
lactose nonfermenters. Others—including certain strains of E. coli, S. sonnei, H. alvei,
Serratia marcescens, and some Yersinia—appear to be lactose nonfermenters because
they lack the permease enzyme that actively transports lactose across the cell
membrane. However, true lactose nonfermenters do not possess β-galactosidase. The
test for β-galactosidase uses the substrate o-nitrophenyl-β-galactopyranoside. At an
alkaline pH, β-galactosidase hydrolyses the substrate, forming o-nitrophenol, which
turns the medium yellow.

Answer 67

A

D The Kaufmann-White schema groups Salmonellae on the basis of the somatic O
(heat-stable) antigens and subdivides them into serotypes based on their flagellar H
(heat-labile) antigens. The Vi (or K) antigen is a capsular polysaccharide that may be
removed by heating. In the United States, commercially available slide agglutination
tests utilize polyvalent antisera (A, B, C-1, C-2, D, E, and Vi) to group the Salmonella
spp. because 95% of isolated organisms belong to groups A through E with the Vi
antiserum identifying the capsular or “K” antigen of Salmonella serotype typhi.
Antiserum is also used against the flagella (“H”) antigens. If the Vi antigen is positive
and the O antigen is negative, the isolate must be heated in boiling water for 15
minutes to remove the capsule and retested with the antisera. There are over 2,200
serotypes of Salmonella.

Answer 68

A

A The drugs of choice for the Enterobacteriaceae species vary, and several genera
display patterns of resistance that aid in their identification. K. pneumoniae and
Citrobacter diversus are resistant to ampicillin and carbenicillin; most Enterobacter
spp. and Hafnia are resistant to ampicillin and cephalothin. Proteus, Morganella, and
Serratia are resistant to colistin. Providencia and Serratia are resistant to multiple
drugs. Several genera are resistant to chloramphenicol and most are resistant to
penicillin.

Answer 69

A

B Strains of E. coli that produce one or both of the Shiga-like toxins (SLT I and SLT II)
can cause bloody diarrhea (hemorrhagic colitis). In the United States, E. coli strain
O157:H7 is the serotype most often associated with hemorrhagic colitis.

Answer 70

A

A Approximately 15 cases of Y. pestis infection are confirmed in the United States
annually. Most originate in the Southwest. It is necessary to be aware of this regional
occurrence because untreated cases are associated with a mortality rate of
approximately 60%. Y. pestis is not fastidious and grows well on blood agar. It is
inactive biochemically, and this helps differentiate it from the other Enterobacteriaceae
species. Y. pestis is the only member of the Enterobacteriaceae family that infects
humans via an insect vector (animal transmission by a flea bite).

Answer 71

A

D K. pneumoniae and E. cloacae display similar IMViC (indole, MR, VP, and citrate)
reactions (00++) and TSI results. However, approximately 65% of E. cloacae strains
are urease positive compared with 98% of those of K. pneumoniae. Enterobacter spp.
are motile, and Klebsiella are nonmotile. The antibiotic pattern of resistance to
carbenicillin and ampicillin is characteristic of the nonmotile Klebsiella spp.

Answer 72

A

D
Typically, the IMViC reactions for the organisms listed are:
E. coli: (++00) (PD = Neg)
S. typhi: (0+00)
Y. enterocolitica: (V+00)
P. vulgaris: (++00) (PD = +)

Answer 73

A

A The most likely organism is a species of Shigella. Typically, Salmonella spp. produce H2S-positive colonies that display black centers on the differential media (except on
MacConkey agar). The biochemical tests listed are necessary to differentiate Shigella
from E. coli because some E. coli strains cross-react with Shigella-typing sera. Shigella
spp. are one of the most common causes of bacterial diarrhea; group D (S. sonnei) and
group B (S. flexneri) are the species most often isolated.

Answer 74

A

B S. marcescens has been implicated in numerous nosocomial infections and is
recognized as an important pathogen with invasive properties. Gelatin hydrolysis and
DNase are positive for both the Proteus spp. and Serratia, but the negative urease and
phenylalanine deaminase are differential. E. cloacae does not produce DNase,
gelatinase, or lysine decarboxylase but will display red-pigmented colonies on
MacConkey agar.

Answer 75

A

A Although P. mirabilis is more frequently recovered from patients with UTIs, P.
vulgaris is commonly recovered from immunosuppressed patients. P. mirabilis is
indole negative and ornithine decarboxylase positive but otherwise is very similar to P.
vulgaris

Answer 76

A

C Most members of the Enterobacteriaceae family produce detectable growth on
MacConkey agar within 24 hours. Y. enterocolita produces non–lactose-fermenting
colonies on MacConkey agar, salmon-colored colonies on Hektoen agar, and yellow or
colorless colonies on XLD agar. If Y. enterocolitica is suspected, specialized agar
(CIN) is employed. The typical “bull’s eye” colonies, dark red with a translucent
border, can be confused with Aeromonas spp. that appear similarly on CIN agar. To
differentiate, an oxidase test must be performed because Yersinia spp. are oxidase
negative and Aeromonas spp. are oxidase positive

Answer 77

A

C Inflammation with bleeding of the mucosa of the large intestine (hemorrhagic colitis)
is a result of an enterohemorrhagic E. coli (EHEC) infection associated with certain
serotypes, such as E. coli O157:H7. The source of the E. coli infection is from
ingestion of undercooked ground beef contaminated with fecal matter or drinking raw
milk.

Answer 78

A

A Y. pestis is the cause of bubonic and pneumonic plague. Bubonic plague causes
swelling of the groin lymph nodes (bubos), whereas pneumonic plague involves the
lungs. The infection caused by bubonic plague may result in fulminant bacteremia that
is usually fatal. The transmission is from rodents (rats, ground squirrels, or prairie
dogs) to humans by the bite of fleas (vectors) or by ingestion of contaminated animal
tissues. Pneumonic plague is acquired via the airborne route when there is close
contact with other pneumonic plague victims.

Answer 79

A

B Preliminary serological grouping of the Salmonella spp. and Shigella spp. should be
performed because reliable commercial polyvalent antisera are available. Sorbitolnegative
(MacConkey agar with sorbitol) colonies of E. coli should be tested by using
commercially available antisera for somatic “O” antigen 157 and flagellar “H” antigen
7. However, Y. pestis isolates should be sent to a public health laboratory for testing
because clinical laboratories generally do not have the typing sera available.

Answer 80

A

D Bubonic plague involves an inflammatory swelling of the lymph nodes of the axilla
and groin, whereas pneumonic plague is associated with an airborne route involving
the lungs. Both infections are caused by the same member of the Enterobacteriaceae
family—Y. pestis.

Answer 81

A

A Point mutations occur in most members of the Enterobacteriaceae family that result in
production of a β-lactamase that hydrolyzes broad-spectrum antibiotics, such as the
cephalosporins, as well as penicillin and monobactam antibiotics. These are known as
ESBL producers. The most common ESBL organisms are K. pneumoniae and E. coli.
ESBL strains are detected by demonstrating their resistance to β-lactam antibiotics.

Answer 82

A

D Serotyping for grouping of Salmonella spp., Shigella spp., and E.coli 0157:H7 using
commercially available slide agglutination test kits is done by most clinical
laboratories. Nucleic acid–based studies are usually performed in reference
laboratories if the demand is minimal in a clinical laboratory. MALDI-TOF MS is used
for the identification of Enterobacteriaceae species. Microbial genomic research
showed that many mass spectral peaks represented ribosomal proteins when compared
with 16S rRNA gene sequencing and could be used to identify clinically important
bacteria (Enterobacteriaceae) with the incorporation of mass spectral libraries and
software. One limitation has been identified in that MALDI-TOF MS cannot
differentiate between E. coli and Shigella spp. Species-level identification compared
with the reference method showed optimal performance in the case of many of the Enterobacteriaceae.

Answer 83

A

D S. sonnei (Serotype D) is the most isolated species of Shigella in the United States.
Humans are the only reservoirs, and of the estimated 500,000 cases of shigellosis in the
United States per year, S. sonnei is responsible for 85% of the infections. It is primarily
a pediatric infection, with 60% seen in children under 10 years of age.

Answer 84

A

A E. coli (STEC) is the organism referred to as enterohemorrhagic E. coli, which
produces a toxin similar to the Shiga toxin produced by S. dysenteriae. Inflammation
and bleeding of the mucosa of the large intestine can lead to kidney failure and HUS.
HUS is characterized by low platelet counts and hemolytic anemia, leading to
transfusions and dialysis and death. The two most common serotypes are 0157:H7 and
0157:NM (nonmotile). Unlike in dysentery, the wet preps of the stools usually lack
white blood cells (WBCs).

Answer 85

A

D P. shigelloides, now considered a new member of the Enterobacteriaceae family,
causes gastrointestinal infections. Research shows that it may cross-react with Shigella
grouping antisera (Group D) causing misidentification. If an oxidase test is performed
first, this error can be avoided (Shigella = oxidase negative, whereas Plesiomonas =
oxidase positive).

Answer 86

A

C NFB will grow on the slant of TSI or KIA, but they do not acidify the butt (glucose
fermentation), as do the Enterobacteriaceae. NFB can be cytochrome oxidase positive
or negative, but all the Enterobacteriaceae species, except Plesiomonas shigelloides,
are oxidase negative. Enterobacteriaceae species grow well on MacConkey agar and
reduce nitrate to nitrite, but NFB grow poorly or not at all on MacConkey and most do
not reduce nitrate. Nearly 70% of NFB recovered from clinical specimens are strains of P. aeruginosa, Acinetobacter spp., or Stenotrophomonas maltophilia

Answer 87

A

A The use of OF tubes helps to determine the presumption of a nonfermentative bacillus
(glucose oxidation positive and glucose fermentation negative). The positive
cytochrome oxidase test and pigment production indicate a possible Pseudomonas
species. Several NFB produce pigments that aid in species identification: P.
aeruginosa produces yellow pyoverdins (fluorescein) and/or pyocyanin (blue aqua
pigment). The characteristic grapelike odor of aminoacetophenone and growth at 42°C
are characteristic of P. aeruginosa.

Answer 88

A

D Both organisms are oxidase positive, motile, and produce pyoverdin. Both are negative
for ONPG and DNase. The differentiating tests are:
Test P. aeruginosa P. putida
Mannitol + Neg
Reduction of + Neg
nitrate to nitrite
42°C growth + Neg

Answer 89

A

A Acinetobacter spp. are nonmotile rods, which appear as coccobacillary forms in
clinical specimens. All are oxidase negative and catalase positive. P. aeruginosa
reduces nitrate to nitrite, whereas Acinetobacter spp. do not. The genus Acinetobacter
is divided into two groups: saccharolytic or glucose-oxidizing species and
asaccharolytic or nonglucose species.

Answer 90

A

B The two genera, Acinetobacter and Alcaligenes, are very similar. Both use oxidation
for the metabolism of carbohydrate, with some strains being nonsaccharolytic. Both
grow well on MacConkey agar. However, Acinetobacter is nonmotile and oxidase
negative. Alcaligenes is motile by peritrichous flagella and oxidase positive.

Answer 91

A

C The two genera, Pseudomonas and Stenotrophomonas, are motile and grow well on
MacConkey agar. However, P. aeruginosa is oxidase positive and grows at 42°C but is
motile only by polar monotrichous flagella. S. maltophilia is oxidase negative, does not
grow at 42°C, and is motile by a polar tuft of flagella

Answer 92

A

B P. aeruginosa is often recovered from the respiratory secretions of patients with CF. If
the patient is chronically infected with the mucoid strain of P. aeruginosa, the
biochemical identification is very difficult. The mucoid strain results from production
of large amounts of alginate, a polysaccharide that surrounds the cell.

Answer 93

A

A P. stutzeri produces dry, wrinkled colonies that are tough and adhere to the media as
well as smooth colonies. B. pseudomallei produces similar colony types but is
distinguished by biochemical tests and susceptibility to the polymyxins. The colonies
of P. stutzeri are buff to light brown because of the relatively high concentration of
cytochromes.

Answer 94

A

D B. pseudomallei produces wrinkled colonies resembling P. stutzeri. Infections are
usually asymptomatic and can be diagnosed only by serological methods. The
organism exists in soil and water in an area of latitude 20° north and south of the
equator (mainly in Thailand and Vietnam). Thousands of U.S. military personnel were
infected with these bacteria during the 1960s and 1970s. The disease may reactivate
many years after exposure and has been called the “Vietnamese time bomb.”

Answer 95

A

A Both organisms produce yellowish pigment and have polar tuft flagella, but the
oxidase and DNase tests are differential.
Test B. cepacia S. maltophilia
Pigment on BAP Green-yellow Lavender-green
Oxidase + Neg
DNase Neg +
Motility + +
Glucose OF (open) + +
Maltose OF (open) + +
Lysine decarboxylase + +

Answer 96

A

D The oxidase test and red pigment (pyorubin), as well as growth at 42°C, distinguish P.
aeruginosa from the other nonfermenters listed, particularly B. cepacia, which is also
associated with CF.

Answer 97

A

A Alcaligenes spp. and Bordetella spp. (nonpertussis) are two organisms that are very
similar biochemically, but B. bronchiseptica is urease positive. Both organisms are
oxidase positive, grow on MacConkey agar, and are motile by peritrichous flagella. B.
bronchiseptica grows well on MacConkey agar, but other species of Bordetella are
fastidious gram-negative rods. B. bronchiseptica is the cause of mild respiratory
infections, whereas B. pertussis is the cause of whooping cough

Answer 98

A

A Chryseobacterium spp. and Acinetobacter spp. often produce a yellow pigment on
blood or chocolate agar and are nonmotile. Acinetobacter spp. are oxidase negative,
grow on MacConkey agar, and are coccobacillary on the Gram-stained smear. In
contrast, Chryseobacterium spp. are oxidase positive, do not grow on MacConkey
agar, and are typically rod shaped. Elizabethkingia (formerly Chryseobacterium)
meningosepticum is highly pathogenic for premature infants. The organism is
transmitted to the neonate via the birth canal and is seen in outbreaks of neonatal units
as well as long-term care facilities

Answer 99

A

D Moraxella spp. are oxidase positive and nonmotile, and this distinguishes them from
Acinetobacter spp. and most Pseudomonas spp. Moraxella spp. are highly sensitive to
penicillin, but Acinetobacter spp. and Pseudomonas spp. are penicillin resistant. M.
lacunata is implicated in infections involving immunosuppressed patients. M. lacunata
is usually associated with eye infections but can also be the cause of bacteremia and
respiratory infections in the immunosuppressed patient.

Answer 100

A

B Growth on Cetrimide (acetyl trimethyl ammonium bromide) agar is used for the
isolation and presumptive identification of P. aeruginosa. With the exception of P.
fluorescens, the other pseudomonads are inhibited along with related nonfermentative
bacteria. Colonies of P. aeruginosa appear as yellow-green to blue-green colonies on
Cetrimide agar.

Answer 101

A

C P. aeruginosa is a cause of a significant number of burn wound infections; these
organisms can exist in distilled water and underchlorinated water. Acinetobacter spp.
are oxidase negative and Moraxella spp. are highly susceptible to penicillin, and this
helps rule them out as possible causes

Answer 102

A

D All species of Acinetobacter are oxidase negative, are nonmotile, and grow on MacConkey agar. Elizabethkingia (formerly Chryseobacterium) spp. produce yellow
pigment (like some Acinetobacter) but are oxidase positive and do not grow well on
MacConkey agar. B. cepacia also produces a yellow pigment but is motile. P.
aeruginosa is motile and grows on Mac agar

Answer 103

A

A In the nitrate test, nitrites formed by bacterial reduction of nitrates will diazotize
sulfanilic acid. The diazonium compound complexes with α-naphthylamine, forming a
red product. Media containing nitrates are used for the identification of nonfermenters.
When testing nonfermenters, it is wise to confirm a negative reaction using zinc dust.
The diazonium compound detects nitrite only, and the organism may have reduced
nitrates to nitrogen, ammonia, nitrous oxide, or hydroxylamine. Zinc ions reduce
residual nitrates in the media to nitrites. A red color produced after addition of zinc
indicates the presence of residual nitrates, confirming a true negative reaction. If a red
or pink color does not occur after adding zinc, then the organism reduced the nitrate to
a product other than nitrite, and the result is considered positive

Answer 104

A

A S. putrefaciens produces abundant H2S on KIA or TSI. Shewanellae are the only
nonfermenters that produce H2S on these media.

Answer 105

A

A B. cepacia (93%) are weakly oxidase positive and motile. Chryseobacterium spp. are
oxidase positive but are nonmotile.

Answer 106

A

B P. aeruginosa is an opportunistic organism that is not part of the human normal flora.
Contact lens solution contamination, eye injury, or contact lens eye trauma are factors
that contribute to P. aeruginosa eye infections. The characteristic blue-green pigment
on Mueller-Hinton agar (pyocyanin pigment) produced by P. aeruginosa and the high
resistance to antibiotics aid in its identification.

Answer 107

A

D P. aeruginosa is highly resistant to many antimicrobial drugs as well as being one of
the most often cultured opportunistic organisms. This virulence factor allows for many
nosocomial infections, such as UTI, wounds (in burn patients), bacteremia, respiratory
tract infection, and CNS infection

Answer 108

A

A B. mallei is rarely transmitted to humans. It is the causative agent of glanders in
mules, donkeys, and horses. It is not part of the human skin flora, and the most likely
transmission to humans is through broken skin

Answer 109

A

B B. pseudomallei infections often produce abscesses in organs (liver, spleen, lungs) as
well as on the skin, in soft tissue, and in joints and bones. Vietnam War veterans
especially may harbor these organisms, which are limited to tropical and subtropical
environments (Southeast Asia and Australia). The organism may surface years later after
surviving in a latent state within phagocytes. There is a high prevalence of this organism
on the surfaces of rice paddies in northern Thailand.

Answer 110

A

D Patients with CF usually do not escape P. aeruginosa infections completely. P.
aeruginosa produces alginate that accounts for the “wet, mucoidy” appearance of colonies. This overproduction of alginate is thought to cause the inhibition of
phagocytosis. The light blue color results from lower production of polyamine, and the
wet or mucoidy appearance is caused by overproduction of alginate. The result is chronic
infections in patients with CF with the “wet” form of P. aeruginosa.

Answer 111

A

A Acinetobacter spp. often colonize immunocompromised patients through intravenous
or urinary catheters. Acinetobacter spp. are nonmotile, oxidase negative, coccobacilli,
which grow well on MacConkey agar.

Answer 112

A

B S. maltophilia are oxidase negative, whereas all the other choices are oxidase positive
(B. gladioli are weakly oxidase positive).

Answer 113

A

C The growth of yellow or green colonies on the selective TCBS agar is dependent on
whether the organism ferments sucrose (producing yellow colonies). Vibrio spp. also
grow well on 5% sheep blood, chocolate, and MacConkey agars. Enrichment with
alkaline peptone broth, pH 8.4, helps recover Vibrio spp. from stool specimens.

Answer 114

A

B V. parahaemolyticus appear as green colonies on TCBS agar, whereas V. cholerae
appear as yellow colonies on TCBS. V. cholerae is the only Vibrio species that causes
a positive string test result. In the test, a loopful of bacterial colonies is suspended in
sodium deoxycholate, 0.5%, on a glass slide. After 60 seconds, the inoculating loop is
lifted out of the suspension. V. cholerae forms a long string resembling a string of
pearls. Salmonella spp. and Shigella spp. are oxidase negative

Answer 115

A

B The only Campylobacter spp. that hydrolyze hippurate are C. jejuni and subsp. doylei.
However, some strains of P. aeruginosa grow on agar selective for Campylobacter at
42°C. C. fetus usually will not grow at 42°C but will grow at 25°C and 37°C.

Answer 116

A

A Campylobacter spp. are best recovered in a microaerophilic atmosphere (reduced O2).
The use of a CO2 incubator or candle jar is not recommended because the amount of
O2 and CO2 do not permit any but the most aerotolerant Campylobacter to survive.
Cultures for Campylobacter should be incubated for 48 to 72 hours before reporting no
growth.

Answer 117

A

D H. pylori is found in specimens from gastric secretions and biopsies and has been
implicated as a cause of gastric ulcers. It is found only in the mucous-secreting
epithelial cells of the stomach. Both H. pylori and C. jejuni are catalase and oxidase
positive. However, Helicobacter spp. are urease positive, which differentiates them
from Campylobacter spp.
Test H. pylori C. jejuni
Nitrate reduction Neg +
Hippurate hydrolysis Neg +
Urease + Neg
Cephalothin sensitivity Sensitive Resistant
Nalidixic acid sensitivity Resistant Sensitive

Answer 118

A

C Aeromonas hydrophilia and other Aeromonas spp. have been implicated in acute
diarrheal disease as well as cellulitis and wound infections. Infections usually follow
exposure to contaminated soil, water, or food. Aeromonas growing on enteric media
are differentiated from the Enterobacteriaceae species by demonstrating that colonies
are oxidase positive. The Aeromonas are sometimes overlooked as pathogens because
most strains grow on selective enteric agar as lactose fermenters.

Answer 119

A

A P. shigelloides is an NLF that will resemble Shigella spp. on MacConkey agar. Both
are TSI Alk/Acid and urease negative. Plesiomonas produces indole and Shigella
usually causes delayed production of indole. However, Plesiomonas is oxidase
positive, whereas Shigella spp. are oxidase negative. P. shigelloides has been added to
the Enterobacteriaceae family through the use of nucleic acid–based methods and is the
only member that is oxidase positive

Answer 120

A

B Both these bacteria cause diarrhea, grow well on enteric agar, and may be confused
with other pathogenic gram-negative rods. Both organisms are positive for oxidase,
motility, indole, and lysine decarboxylase. The following reactions are differential:
Test A. hydrophilia P. shigelloides
β-Hemolysis + Neg
on sheep
blood agar
DNase + Neg
Voges-Proskauer + Neg

Answer 121

A

A Aggregatibacter spp. (formerly Actinobacillus spp.) and formerly Centers for Disease
Control and Prevention (CDC) groups HB-3 and HB-4 share many biochemical
characteristics of the Haemophilus spp. Infections most often associated with this
gram-negative coccobacillus are subacute bacterial endocarditis and periodontal
disease (its main habitat is the mouth). The most common human isolate is
Aggregatibacter (formerly Actinobacillus) actinomycetemcomitans, which grows
slowly on chocolate agar. It is positive for catalase, nitrate reduction, and glucose
fermentation. It does not grow on MacConkey agar and is negative for oxidase, urease,
indole, X, and V requirements.

Answer 122

A

B C. hominis (indole positive) is a gram-negative coccobacillus biochemically similar to
Aggregatibacter (Actinobacillus) spp. (indole negative). Like Aggregatibacter
(formerly Actinobacillus) spp., it is a cause of endocarditis. However, Cardiobacterium
spp. are positive for cytochrome oxidase and negative for nitrate reduction and
catalase, whereas most Aggregatibacter (formerly Actinobacillus) spp. are negative for
oxidase and positive for nitrate reduction and catalase. C. hominis will grow on blood
agar after 48 to 72 hours in 5% CO2 at 35°C, but Aggregatibacter (formerly
Actinobacillus) requires chocolate agar.

Answer 123

A

A E. corrodens is a part of the normal flora of the upper respiratory tract and the mouth.
It is often seen after trauma to the head and neck, dental infections, and human bite
wounds. It requires blood for growth. The organism causes pits in the agar, where
colonies are located. The smell of bleach may be apparent when the plates are
uncovered for examination. Aggregatibacter (formerly Actinobacillus) spp. and C.
hominis both utilize several carbohydrates, and Proteus spp. are oxidase negative

Answer 124

A

A Both K. kingae and E. corrodens are gram-negative rods that are oxidase positive and
catalase negative. Both grow well on blood and chocolate agars and cause pitting of
the media, and neither grows on MacConkey or XLD agar. However, K. kingae strains
produce a narrow zone of β-hemolysis on sheep blood agar similar to that of group B
streptococci.

Answer 125

A

B Kingella spp. are gram-negative coccobacilli or plump-looking rods. They are part of
the normal flora of the upper respiratory and urogenital tracts of humans. Infection is
seen primarily in patients having underlying heart disease, poor oral hygiene, or
iatrogenic mucosal ulcerations (e.g., radiation therapy), in whom the organism is
recovered from blood cultures.

Answer 126

A

C The Capnocytophaga species C. gingivalis, C. sputigena, and C. ochracea are part of
the normal oropharyngeal flora of humans; however, C. canimorsus and C. cynodegmi
(formerly CDC groups DF-2 and DF-2-like bacteria) are associated with infections
resulting from dog bite wounds and cat bites as well as scratch wounds.

Answer 127

A

B Pasteurella multocida (P. canis) is part of the normal mouth flora of cats and dogs
and is frequently recovered from wounds inflicted by them. It produces large amounts
of indole and therefore an odor resembling that of colonies of E. coli. Pseudomonas
spp. are also catalase and oxidase positive but can be ruled out because it grows on
MacConkey agar and does not produce indole.

Answer 128

A

D B. pertussis is an oxidase-positive, nonmotile, gram-negative coccobacillus and
appears as small, round colonies resembling droplets of mercury on Bordet-Gengou
media with sheep blood agar. It is fastidious and does not grow on chocolate or
MacConkey agar. However, B. pertussis adapts to blood agar, growing within 3 to 6
days. This organism is the cause of whooping cough, which can be prevented by
immunization with the diphtheria, tetanus, pertussis (DPT) vaccine. The DPT vaccine
contains diphtheria and tetanus toxoids and killed whole-cell B. pertussis.

Answer 129

A

A Although blood agar will support the growth of Brucella spp., Castenada bottles are
the medium of choice. Castenada bottles contain a slant of enriched agar medium that
is partially submerged and surrounded by an enriched broth medium. As the specimen
is injected into the bottles and mixed, the agar slant is simultaneously coated with the
blood (or bone marrow). Brucella is the cause of undulant fever (acquired by ingesting
infected, unpasteurized milk products) and is responsible for many cases of fever of
unknown origin. Brucella spp. are facultative intracellular (multiply in phagocytic
cells) organisms and grow very slowly, usually requiring 4 to 6 weeks for recovery.
Brucella melitensis is the most frequently recovered species.

Answer 130

A

A B. abortus can be differentiated from B. melitensis by the following reactions:
H2S Urease CO2 Requirement Basic Fuchsin Thionin (20 mg)
B. melitensis Neg V Neg + growth + growth
B. abortus + + +/V + growth (Neg) No growth

Answer 131

A

C F. tularensis (the causative agent of tularemia) is a fastidious gram-negative rod that
is best recovered from lymph node aspirates and tissue biopsies. It is oxidase negative,
nonmotile, and inert biochemically. Cysteine blood agar is the medium of choice, but
F. tularensis will grow on commercially prepared chocolate agar because it contains X
factor and is supplemented with a growth enrichment (IsoVitaleX) that contains
cysteine. F. tularensis may not grow well on MacConkey agar.

Answer 132

A

D L. pneumophila should be recovered on buffered CYE agar. This agar is nonselective
but can be made more selective for Legionella spp. by addition of the antibiotics
cefamandole, polymixin B, and anisomycin. Any small, glistening, convex colonies on
buffered CYE agar after 2 to 3 days of incubation that do not grow on L-cysteine–
deficient buffered CYE agar or routine nonselective media should be further tested by
the direct fluorescent antibody (DFA) test for confirmation of L. pneumophila. Rapid
detection by using enzyme immunoassay (EIA) and immunochromotographic methods
in tissue and urine has proven successful, as well as polymerase chain reaction (PCR)
for respiratory specimens.

Answer 133

A

A Both H. influenzae and H. aegyptius are glucose, urease, oxidase, and catalase
positive. H. influenzae (biotype II) is positive for both indole and xylose, whereas H.
aegyptius is negative for both tests. Biotype II encompasses 40% to 70% of H.
influenzae strains recovered from clinical specimens. H. aegyptius is responsible for
epidemics of conjunctivitis in children.

Answer 134

A

C The V factor, NAD, must first be released from RBCs before it can be assimilated by
Haemophilus spp. Chocolate agar is made by heating blood agar to lyse RBCs. The
released NAD is directly available to those Haemophilus species requiring it.
Chocolate agar also contains the X factor (hemin). All Haemophilus except H. ducreyi
and H. aphrophilus (now called Aggregatibacter aphrophilus) require V factor,
whereas X factor is required by H. influenzae, H. haemolyticus, and H. ducreyi.

Answer 135

A

A Staphylococcus growing on sheep blood agar produce NAD (V factor) as a metabolic
by-product and β-hemolysins which lyse sheep RBCs, releasing hemin (X factor). This
satellite feature allows pinpoint-sized colonies of Haemophilus spp. to grow around
Staphylococcus colonies. Sheep blood agar alone does not support the growth of
Haemophilus spp., which require V factor because of the presence of V factor–
inactivating enzymes that are present in the agar.

Answer 136

A

B Production of β-hemolysis is used to distinguish these two species from other
Haemophilus with the same X and V requirements. Horse blood agar furnishes X
factor and, when supplemented with yeast extract, supports the growth of Haemophilus
spp. Sheep blood agar is not used because it contains growth inhibitors for some
Haemophilus spp. The chart at the top of the page summarizes the characteristics of the
Haemophilus spp.

Answer 137

A

B The majority of H. influenzae infections occur in children under 5 years of age and
are caused by capsular serotype b (Hib), one of six serotypes designated a through f.
This capsular strain appears to contain a virulence factor that makes it resistant to
phagocytosis and intracellular killing by neutrophils. The nontypable strains do not
have a capsule and are part of the normal respiratory flora. Serotyping of Haemophilus
is performed by mixing colonies with agglutinating antibodies available as commercial
agglutination kits. Identification is also done by using PCR.

Answer 138

A

C A. (H.) aphrophilus does not require either X or V factor for growth and is
differentiated from the other Haemophilus species by its ability to produce acid from
lactose and a positive ALA test. H. influenzae and H. haemolyticus are incapable of
synthesizing protoporphyrin from Δ-ALA and are negative for this test.

Answer 139

A

B H. ducreyi requires exogenous X factor and causes genital lesions referred to as “soft
chancres.” It is difficult to grow and requires commercial chocolate agar or
gonococcus base medium containing 1% to 2% hemoglobin, 5% fetal calf serum, and
1% IsoVitaleX enrichment. The plates must be incubated at 33oC in a 3% to 5% CO2
moist environment for 2 to 3 days. Recovery of H. ducreyi from specimens constitutes
a reportable sexually transmitted disease (chancroid).

Answer 140

A

A Roughly 20% of H. influenzae strains produce β-lactamase, which hydrolyses and
inactivates the β-lactam ring of ampicillin (and penicillin).

Answer 141

A

B H. influenza is the main cause of meningitis in young, unvaccinated children.
Although several biotypes of H. parainfluenzae produce indole and urease, H.
parainfluenzae does not require X factor for growth. H. ducreyi requires X factor, but
not V factor. A. (H.) aphrophilus does not require either X factor or V factor for
growth.

Answer 142

A

A The X factor requirement for growth is the cause of many inaccuracies when
identifying Haemophilus spp. requiring this factor. False-negative results have been
attributed to the presence of small amounts of hemin in the basal media, or X factor
carryover from colonies transferred from primary media containing blood. The ALA
test determines the ability of an organism to synthesize protoporphyrin intermediates in
the biosynthetic pathway to hemin from the precursor compound Δ-ALA.
Haemophilus species that need exogenous X factor to grow are unable to synthesize
protoporphyrin from Δ-ALA and are negative for the ALA test. These include H.
influenzae, H. haemolyticus, H. aegyptius, and H. ducreyi.

Answer 143

A

A Bartonella spp., frequently the cause of zoonoses infections, are difficult to grow on
primary culture media because they are facultative intracellular organisms. Bartonella
henselae is the organism found in domestic cats, the main reservoir. When CSD is
suspected from the patient’s history, blood cultures should be smeared and Gram
staining performed. Bartonella spp. are biochemically inert—that is, they are negative
for oxidase, catalase, indole, and urease tests. Therefore, commercial identification
systems, DNA amplification for various genes, and indirect immunofluorescence
assays (IFAs) are used to identify these organisms

Answer 144

A

B B. pertussis, the cause of whooping cough, is highly contagious during the 5- to 10-
day period after acquisition. The incidence of whooping cough is greater in
nonimmunized individuals, and therefore, is higher in children ages less than 1 year. B.
bronchiseptica is only rarely found in humans, but may cause respiratory disease in
animals. Unlike B. pertussis it is positive for nitrite, urease, and motility. A direct
identification by DFA using polyclonal antibody to detect B. pertussis in smears from
nasopharyngeal material is the quickest method and less costly compared with PCR.

Answer 145

A

D Persons handling samples suspected of containing F. tularensis must wear gloves and
use a biological safety cabinet (follow biosafety Level [BSL]-2 controls). For cultures,
BSL-3 controls must be followed. Tularemia (rabbit fever or deer fly fever) is one of
the most common laboratory-acquired infections, and it is recommended that
specimens be sent to a reference laboratory for identification and serological testing. F.
tularensis requires cysteine and cystine to grow. It may grow on chocolate agar
supplemented with IsoVitaleX and also on charcoal yeast extract agar used to isolate
Legionellae

Answer 146

A

B Elizabethkingia (formerly Chryseobacterium) meningosepticum can cause septicemia
and meningitis in neonates and immunocompromised adults. The ability to
encapsulate, produce proteases, and survive in chlorinated tap water are factors that
contribute to hospital-acquired infections with this bacterium.

Answer 147

A

A C. canimorsus are part of the oral flora of dogs. The organisms require at least 5%
CO2 for growth and grow slowly on blood and chocolate agars. Colonies can grow in
48 hours if cultured in high CO2 on BHI agar with 5% sheep blood.

Answer 148

A

B Blood cultures growing small, gram-negative rods should alert the microbiologist to
the possibility of infection with one of the five AACEK (formerly HACEK)
organisms. Although responsible for less than 5% of bacterial endocarditis overall,
greater than 50% of endocarditis cases caused by gram-negative rods result from one
of them.

Answer 149

A

D Legionella spp. stain poorly, if at all, with Gram stain. Legionella pneumophilia is not
acid fast, although L. micdadei, which accounts for a small percentage of Legionella
pneumonia infections, is acid-fast positive. Specimens suspected of containing
Legionella spp. should be handled in a Class II biological safety cabinet. Legionella
spp. require buffered charcoal–yeast extract (BCYE) agar for growth and will not grow
on MacConkey agar. Because culture can take up to 10 days, rapid diagnosis by direct
immunofluorescence (DIF) and DNA amplification are preferred. Direct fluorescent
antibody (DFA) tests are not as sensitive as culture or PCR but are specific and can be
used to rapidly confirm a positive serological test, which may be positive in the
absence of disease. MALDI-TOF MS can be used to identify isolates, but this method
cannot distinguish serotypes. PCR methods using respiratory specimens directly,
(without culture for the identification of Legionella spp.) are 99% specific and 85%
sensitive.

Answer 150

A

A Tissue specimens are immersed in urea broth or placed in urea agar. A positive test
indicates the presence of urease produced by H. pylori. Another method is the urea
breath test, which is less invasive

Answer 151

A

B AACEK (formerly HACEK) denotes the following slow-growing gram-negative
bacilli causing subacute bacterial endocarditis: Aggregatibacter aphrophilus;
Aggregatibacter actinomycetemcomitans; Cardiobacterium hominis; Eikenella
corrodens; and Kingella kingae

Answer 152

A

B H. ducreyi may take up to 7 days to grow on chocolate agar (requiring X factor for
growth) and preferably at 33°C with high humidity. Testing for chancroid disease
requires specialized media

Answer 153

A

C To separate C. jejuni subsp. jejuni from C. jejuni subsp. doylei (both are positive for
hippurate hydrolysis and grow at 37°C and 42°C), it is best to use cephalothin (30-μg
disk) and nalidixic acid (30-μg disk) antibiotic testing. C. jejuni subsp. jejuni are
resistant to cephalothin and susceptible to nalidixic acid. C. jejuni subsp. doylei are
susceptible to both disks

Answer 154

A

A B. pertussis produces a virulence factor, pertussis toxin (PT). The organism attaches to the respiratory ciliated epithelial cells of the upper respiratory tract, causing
coughing and irritation. The recommended method for identification of the toxin in
serum is by ELISA using paired samples of acute and convalescent sera. PCR on
nasopharyngeal swabs as well as DFA tests are available. MALDI-TOF MS can also
be used successfully for identification

Answer 155

A

B The catalase test (utilizing a 3% hydrogen peroxide [H2O2] solution stored in a brown
bottle under refrigeration) is positive for the Staphylococcus and Micrococcus spp.
Members of the Streptococcaceae family are negative.
Staphylococcus spp. Micrococcus spp.
Open tube
(oxidation)
+ +
Closed tube
(fermentation)
+ Neg

Answer 156

A

A Both micrococci and staphylococci are catalase-positive and gram-positive cocci. On
direct smears, they both appear as pairs, short chains (resembling Streptococcus spp.),
or clusters. However, the micrococci fail to produce acid from glucose under anaerobic
conditions. The OF tube reactions are provided in the table above.

Answer 157

A

C Lysostaphin is an endopeptidase that cleaves the glycine-rich pentapeptide cross bridges in the staphylococcal cell wall peptidoglycan. The susceptibility of the
staphylococci to lysostaphin (less than 200 μg/mL) is used to differentiate them from
the micrococci. Staphylococci are susceptible and show a 10- to 16-mm zone of
inhibition, whereas micrococci are not inhibited and will grow up to the disk.

Answer 158

A

D The slide coagulase test using rabbit plasma with ethylenediaminetetraacetic acid
(EDTA) detects bound coagulase or “clumping factor” on the surface of the cell wall,
which reacts with the fibrinogen in the plasma. This test is not positive for all strains of
S. aureus, and a negative result must be confirmed by the tube method for detecting
“free coagulase” or extracellular coagulase. The tube test is usually positive within 4
hours at 35°C; however, a negative result must then be incubated at room temperature
for the remainder of 18 to 24 hours. Some strains produce coagulase slowly or produce
fibrinolysin, which dissolves the clot at 35°C. Latex agglutination procedures utilize
fibrinogen and immunoglobulin G (IgG)-coated latex beads that detect protein A on
the staphylococcal cell wall

Answer 159

A

B In addition to coagulase, the virulence of S. aureus is attributed to hyaluronidase,
which damages the intercellular matrix (basement membrane) of tissues. β-Lactamase–
producing strains are able to inactivate penicillin and ampicillin, making the organism
resistant to these antibiotics. Lecithinase is not produced by S. aureus, and urease is
not a virulence factor.

Answer 160

A

C S. aureus is the organism most often recovered from female patients. These strains
produce toxic shock syndrome toxin 1 (TSST-1). Toxic shock syndrome is attributed
to the use of certain highly absorbent tampons by menstruating females. The toxin is
also recovered from sites other than the genital area and produces fever and lifethreatening
systemic damage as well as shock.

Answer 161

A

A S. intermedius may be falsely identified as S. aureus in dog bite wounds because it
infects mammals and certain birds, but not usually humans. Cases involving humans
result from animal bites and are most often seen in persons who work closely with
animals. S. intermedius may give a positive latex agglutination coagulase test

Answer 162

A

D S. epidermidis represents 50% to 80% of all coagulase-negative Staphylococcus spp.
recovered from numerous clinical specimens. It is of special concern in nosocomial
infections because of its high resistance to antibiotics.

Answer 163

A

B S. epidermidis produces an extracellular slime (exopolysaccharide or biofilm) that
enhances the adhesion of these organisms to indwelling plastic catheters. The slime
production is considered a virulence factor and is associated with infections from
prostheses.

Answer 164

A

A Methicillin-resistant S. aureus (MRSA) and methicillin-resistant S. epidermidis
(MRSE) are called heteroresistant. This refers to two subpopulations in a culture, one
that is susceptible and the other that is resistant to antibiotic(s). The resistant
population grows more slowly than the susceptible one and can be overlooked.
Therefore, the more resistant subpopulation should be promoted growthwise by using a
neutral pH (7.0–7.4), cooler incubation temperatures (30°C–35°C), the addition of 2%
to 4% NaCl, and incubation up to 48 hours. For identification of MRSA, molecular
assays (using amplified DNA) and MALDI-TOF MS (used directly from blood
cultures) have proven successful.

Answer 165

A

B S. saprophyticus is coagulase negative and resistant to 5 μg of novobiocin. When using the standardized Kirby-Bauer sensitivity procedure, a 6- to 12-mm zone of
growth inhibition is considered resistant. Susceptible strains measure 16 to 27 mm
(inhibition) zones

Answer 166

A

C S. aureus can produce fibrinolysins that dissolve the clot formed by the coagulase
enzyme. The tube method calls for an incubation of 4 hours at 35°C to 37°C and 18 to
24 hours at room temperature. Both must be negative to interpret the result as
coagulase negative. This organism is coagulase positive and, therefore, identified as S.
aureu

Answer 167

A

D Vancomycin, along with rifampin, is used for strains of S. aureus that are resistant to
the β-lactams. MRSA strains pose problems when reading the zone sizes for these
strains. Their heteroresistance results in a film of growth consisting of very small
colonies formed within the defined inhibition zone surrounding the antibiotic disk.
Initially, this appears as a mixed culture or contaminant

Answer 168

A

A The production of acetoin by S. aureus from glucose or pyruvate differentiates it from
S. intermedius, which is also coagulase positive (positive for clumping factor and free
coagulase or tube test). This test is the Voges-Proskauer (VP) test. Acetoin production
is detected by addition of 40% KOH and 1% α-naphthol to the VP test broth after 48
hours of incubation. A distinct pink color within 10 minutes denotes a positive test
result.

Answer 169

A

C Micrococcus spp. utilize glucose oxidatively but not under anaerobic conditions
(sealed tube). Staphylococcus spp. utilize glucose oxidatively and anaerobically. The
catalase differentiates the Micrococcaceae family (positive) from the Streptococcaceae
family (negative).

Answer 170

A

D S. epidermidis and S. saprophyticus are the two possibilities because they are both
catalase positive, coagulase negative, urease positive, and ferment lactose. Novobiocin
susceptibility is the test of choice for differentiating these two species. S. epidermidis
is sensitive, but S. saprophyticus is resistant to 5 μg of novobiocin. UTIs caused by S.
saprophyticus are often seen in cultures from sexually active young females.

Answer 171

A

D S. aureus is novobiocin sensitive and cannot be ruled out by a negative clumping
factor test. Most S. aureus produce β-hemolysis on sheep blood agar plates and are
mannitol salt positive (produce acid and are not inhibited by the high salt
concentration). The tube test (to detect free coagulase) should be performed because
the slide test was negative.

Answer 172

A

A Staphylococci are susceptible to furazolidone, giving zones of inhibition that are 15
mm or greater. Micrococcus spp. are resistant to furazolidone, giving zones of 6 to 9
mm. The test is performed as a disk susceptibility procedure using a blood agar plate

Answer 173

A

A A bacitracin disk (0.04 unit) is used to identify group A β-hemolytic streptococci, but it will also differentiate catalase-positive organisms. A zone of 10 mm or greater is
considered susceptible. The Staphylococcus species are resistant and grow up to the
disk, whereas Micrococcus species are sensitive.

Answer 174

A

C The modified oxidase test (Microdase) is used to rapidly identify catalase-positive
gram-positive cocci as Micrococcus spp. (positive) or Staphylococcus spp. (negative).
Filter paper disks that are saturated with oxidase reagent (tetramethyl-pphenylenediamine
in dimethylsulfoxide) are used. A colony of the isolate is rubbed
onto the paper. Oxidase-positive organisms produce a purple color within 30 seconds
or a blue color within 2 minutes if using the commercial product Microdase.

Answer 175

A

B Streptococcus spp. are facultative anaerobes that grow aerobically as well and are
oxidase and catalase negative. To demonstrate streptolysin O on blood agar, it is best
to stab the agar to create anaerobiosis because streptolysin O is oxygen labile.

Answer 176

A

A Group A β-hemolytic streptococci are the cause of scarlet fever, and some strains
produce toxins (pyrogenic exotoxins A, B, and C) that cause a scarlatiniform rash.
Streptococcus pyogenes is the cause of puerperal fever, streptococcal toxic shock,
rheumatic fever, and a severe “flesh-eating” bacterial infection.

Answer 177

A

A The antistreptolysin O (ASO) titer is used to indicate a recent infection with group A
β-hemolytic streptococci. Streptolysin O may also be produced by some strains of
groups C and G streptococci

Answer 178

A

A The bacitracin A disk test is no longer used, but the PYR, hippurate hydrolysis, and
CAMP tests are used for the presumptive identification of the β-hemolytic
streptococci. In addition to group A, groups C, F, and G are also β-hemolytic and give
a positive test result for bacitracin (a zone of inhibition of any size). Therefore, a
positive bacitracin A disc test does not confirm the presence of group A β-hemolytic
streptococci. Group A strep test positive for PYR but negative for hippurate and the
CAMP test. Group B streptococci test negative in the PYR test but positive in the
hippurate and CAMP tests. Groups C, G and F test negative in all three tests.

Answer 179

A

B SXT is inhibitory to most streptococci except S. pyogenes and Streptococcus
agalactiae. For this reason, SXT is used in a commercially available streptococcal
selective agar (SSA) as a primary plating agar for the detection of group A
streptococci. Latex agglutination test kits are used for the rapid detection of β-
hemolytic streptococci Lancefield groups A, B, C, F, and G.

Answer 180

A

D Streptococci that are not group A or B may be either resistant or susceptible to
bacitracin but are usually susceptible to SXT.
β-Hemolytic Streptococci Bacitracin Sulfamethoxazole-Trimethoprim
Group A Susceptible Resistant
Group B Resistant Resistant
Non-A,
non-B
Susceptible
or resistant
Susceptible

Answer 181

A

A The CAMP test (described by Christie, Atkins, and Munch-Petersen in 1944) refers to
a hemolytic interaction (flaming arrowhead) that is seen on a blood agar plate between
the β-hemolysins produced by most strains of S. aureus and an extracellular protein
produced by both hemolytic and nonhemolytic isolates of group B streptococci. In a
CAMP test, the plate must be placed in an ambient air incubator at 35°C to 37°C.
Group A streptococci may be CAMP positive if the plate is incubated in a candle jar,
high CO2 atmosphere, or anaerobically.

Answer 182

A

D The bile esculin test differentiates those bacteria that can hydrolyze esculin and also
grow in the presence of 4% bile salts or 40% bile. The bile esculin slant or plate is
inoculated on the surface and incubated for 24 to 48 hours in a non-CO2 incubator.
Group D streptococci (enterococci and nonenterococci) are positive, causing
blackening of half or more of the slant within 48 hours. Viridans streptococci are
negative (do not grow or hydrolyze esculin).

Answer 183

A

B The bile solubility test can be performed directly by dropping 2% sodium
deoxycholate onto a few well-isolated colonies of S. pneumoniae. The bile salts speed
up the autolysis observed in pneumococcal cultures. The colonies lyse and disappear
when incubated at 35°C for 30 minutes, leaving a partially hemolyzed area on the
plate. The same phenomenon can be seen using a broth culture; addition of 10%
deoxycholate to broth containing S. pneumoniae results in visible clearing of the
suspension after incubation at 35°C for 3 hours.

Answer 184

A

A Optochin (P disk) at a concentration of 5 μg/mL or less inhibits the growth of S.
pneumoniae but not viridans streptococci. However, optochin at a concentration in
excess of 5 μg/mL inhibits viridans streptococci as well if incubated under 5% CO2 at
35°C for 18 to 24 hours. A zone of inhibition of 14 mm or more around the 6-mm disk
is considered a presumptive identification of S. pneumoniae. A questionable zone size
should be confirmed by performing a bile solubility test

Answer 185

A

D E. faecalis will grow in 6.5% salt, and the nonenterococci (S. bovis and S. equinus)
will not. This test distinguishes the enterococci group from S. bovis and S. equinus
(nonenterococci group). Both groups grow on bile esculin agar. Viridans streptococci
are negative for 6.5% salt growth and will not grow on bile esculin agar

Answer 186

A

B Approximately 80% of group B streptococci are capable of growing in 6.5% salt
broth; however, they do not hydrolyze esculin or grow in media containing 4% bile
salts.

Answer 187

A

D A precipitin reaction seen microscopically with methylene blue stain (microprecipitin
reaction) occurs between the carbohydrate of the capsule of S. pneumoniae and
anticapsular antibody. The antibody may be type specific or polyvalent. Binding of
antibodies to the bacteria causes the capsule to swell, identifying the organisms as S.
pneumoniae. Antigen detection kits are used to identify S. pneumoniae recovered from
CSF and blood cultures. S. pneumoniae is the cause of 95% of the bacterial
pneumonias in unvaccinated children and the leading cause of bacterial meningitis

Answer 188

A

A The PYR hydrolysis test is highly specific for group A streptococci and Enterococcus
spp. The test detects the pyrrolidonylarylamidase enzyme, which hydrolyzes PYR

Answer 189

A

D The β-hemolytic streptococci—not of groups A or B—are sensitive to SXT and may
be either sensitive or resistant to bacitracin. Groups A and B are both resistant to SXT.
Group A and E. faecalis are PYR positive. E. faecalis is also positive for bile esculin
and 6.5% salt broth. Latex agglutination tests identify β-hemolytic groups A, B, C, F,
and G

Answer 190

A

C VRE (Vancomycin-Resistant Enterococcus) denotes E. faecalis and Enterococcus
facium, which have shown resistance to most antimicrobials used to treat infections
caused by gram-positive cocci (ampicillin, methicillin and other β-lactams) and
therefore have mutated and now are resistant to vancomycin. Group A streptococci are
sensitive to bacitracin and negative for bile esculin and 6.5% salt broth. Group B
streptococci will grow in 6.5% salt broth but are negative for bile esculin and PYR.
The non–group A, non–group B streptococci will not grow in 6.5% salt broth and are
sensitive to SXT. E. faecalis is positive for bile esculin, 6.5% salt broth, and PYR.

Answer 191

A

D The staphylococcal streak, across the NVS inoculum, provides the nutrients needed.
Very small colonies of NVS can be seen growing adjacent to the staphylococcal streak
on the blood agar plate in a manner similar to the satellite phenomenon of
Haemophilus spp. around S. aureus. NVS are normal flora of the oral cavity but may
cause life-threatening infections when introduced into sterile areas (cause of
endocarditis). Blood culture media will supply the necessary nutrients for their
recovery.

Answer 192

A

A E. faecalis is resistant to penicillin and ampicillin as well as some of the
aminoglycoside antibiotics. Pneumococci, group B streptococci, and S. bovis are PYR
negative

Answer 193

A

B S. bovis and S. salivarius are physiologically and biochemically similar. They are both
PYR and 6.5% salt broth negative and bile esculin positive, but only S. bovis is
positive for mannitol and starch reactions. See the following chart:
Bacitracin PYR BE 6.5%
Salt
Mannitol Starch
S. bovis R Neg + Neg + +
S. salivarius R Neg + Neg Neg Neg

Answer 194

A

B Group B streptococci (S. agalactiae) are resistant to both bacitracin and SXT. Unlike
group A and E. faecalis, the group B streptococci are negative for PYR. With some
exceptions, group B streptococci will grow in 6.5% salt broth. Group B strep infections
in newborns are the result of premature rupture of the mother’s membranes. The
infection in a newborn happens within the first 5 days after birth and results in fever,
sepsis, meningitis, and respiratory distress

Answer 195

A

A Both N. gonorrhoeae and N. meningitidis grow selectively on MTM as a result of the
addition of vancomycin and colistin, which inhibit gram-positive and gram-negative bacteria, respectively. Trimethoprim is added to inhibit swarming of Proteus spp.
because a rectal swab may be used for culture. Nystatin and amphotericin B are used to
prevent growth of yeasts and molds from vaginal specimens. N. lactamica will grow
on MTM but is rarely involved in human infection.

Answer 196

A

D Upon subculture from a primary plate, various sizes and appearances of gonococci
result from multiple nutritional requirements, such as arginine–hypoxanthine–uracil
(AHU)–requiring strains. Colony size and coloration (or light reflection) are the basis
of the Kellogg scheme (types T1 through T5). Types T1 and T2 have pili on the
surface, whereas T3, T4, and T5 do not. Transparent media are not used routinely, but
opaque and transparent colonial differences of the gonococci can be seen when using
it.

Answer 197

A

D All of the listed Neisseria spp. grow on MTM and are oxidase positive. N. lactamica
is a nonpathogenic component of normal throat flora resembling N. meningitidis, but it
grows well on selective MTM agar. Presumptive identification of N. meningitidis or N.
gonorrhoeae is stated only if the source of the specimen (i.e., urogenital or CSF) is
given. The identification must be confirmed by further testing, such as carbohydrate
utilization tests, DNA tests, or rapid latex slide agglutination tests.

Answer 198

A

A N. gonorrhoeae colonies recovered from selective MTM media give an immediate 4+
positive reaction (bubbling) when 30% H2O2 is added. The catalase test uses 3% H2O2.
This is a presumptive test for N. gonorrhoeae. N. meningitidis and N. lactamica give a
weak or delayed bubbling reaction. M. catarrhalis is catalase positive, whereas the
Superoxol reactions are negative, weak, or delayed and it shows a variable growth
pattern on MTM.

Answer 199

A

B Moraxella spp. are oxidase and catalase positive, as are the gonococci. Neisseria spp.
and M. catarrhalis will keep their typical coccal morphology after overnight
incubation on blood agar with a 10-unit penicillin disk (CO2 incubation). Other
Moraxella spp. form long filaments or long spindle-shaped cells when grown near a
10-unit penicillin disk

Answer 200

A

A Gram staining of urethral discharge (in men only) that shows typical gonococcal cells
in PMNs should be reported “presumptive N. gonorrhoeae, confirmation to follow.”
With female patients, the normal vaginal flora contain gram-negative cocci and
diplococci resembling gonococci, and therefore, no presumptive identification should
be reported for N. gonorrhoeae from the vaginal Gram-stained smear.

Answer 201

A

A N. lactamica utilizes lactose by producing the enzyme β-galactosidase. All other
Neisseria spp. that grow on MTM media are lactose negative.

Answer 202

A

C CTA agar with 1% carbohydrate and phenol red pH indicator added is used for the
identification of Neisseria species. CTA carbohydrates must be placed in an ambient
air incubator because a high CO2 concentration may reduce the pH, causing a falsepositive
(acid) result. The utilization of carbohydrates by some fastidious gonococcal
strains may take up to 72 hours to produce a color change in the pH indicator is rarely
used because identifications are done by rapid biochemical, immunologic, or
enzymatic methods as well as with nonamplified DNA probes.

Answer 203

A

D N. lactamica is part of the normal vaginal and throat flora and is the only Neisseria
species that grows on MTM that utilizes lactose. Other saprophytic Neisseria spp. may
utilize lactose but do not grow on MTM media.

Answer 204

A

A M. catarrhalis is part of the normal upper respiratory flora but is implicated in lower
respiratory infections, including pneumonia. It produces stunted growth on MTM and
is DNase positive, characteristics that differentiate it from the other saprophytic
Neisseria spp.

Answer 205

A

A Oxacillin is the drug used to screen staphylococci for resistance to antibiotics having
the β-lactam ring. Included in this group are penicillin, cephalosporin, monobactam,
and carbapenem. MRSA defines strains of staphylococci that are resistant to all of
these antibiotic groups. MRSA strains are treated with vancomycin or oxazolidinone.

Answer 206

A

D The standard Kirby-Bauer method used for disk diffusion susceptibility testing
recommended by the Clinical and Laboratory Standards Institute (CLSI) is 20 mm.
The test requires 20- to 24-hour incubation at 35°C in 5% to 7% CO2. If the zone size
is 19 mm, an MIC should be performed on isolates from CSF or blood.

Answer 207

A

C A test for β-lactamase production should be performed on all isolates of Staphylococcus and Enterococcus as well as N. gonorrhoeae and H. influenzae. Some
organisms may appear to be penicillin/ampicillin susceptible by the conventional
Kirby-Bauer disk diffusion method for β-lactamase producers, but still produce small
amounts of β-lactamase. An example is heteroresistant mecA-positive S. aureus (the
mecA gene confers penicillin resistance and is present in all MRSA strains).

Answer 208

A

B By using a 15-μg erythromycin disk adjacent to a 2-μg clindamycin disk in a disk
diffusion procedure, a flattening of the clindamycin zone occurs in inducible
clindamycin resistance, causing the zone to look like the letter “D.” This is referred to
as being D-zone positive. The CLSI recommends that inducible clindamycin resistant
strains be reported as resistant, with a comment that clindamycin may still be effective
in some patients; however, an alternative therapy is recommended for treatment of the
infection

Answer 209

A

A S. aureus (VRSA) is determined if the MIC result is 16 μg/mL or greater. Strains
showing high levels of resistance (MIC greater than 8 μg/mL) and intermediate
resistance (MIC 4 to 8 μg/mL) are thought to have structural changes within the cell
wall. Two new agents available to combat vancomycin-resistant strains are linezolid
and daptomycin.

Answer 210

A

B Strains of S. aureus producing toxins A and B are the cause of “scalded skin
syndrome,” which refers to skin infections that can be manifested as small blisters or
covering 90% or greater of the body surface in newborns.

Answer 211

A

B The only spore former listed is the Bacillus spp., which grow as large, spreading
colonies on blood agar plates. Pseudomonas spp. are gram-negative rods;
Corynebacterium spp. appear as small, very dry colonies on BAP; Listeria spp. appear
as very small β-hemolytic colonies on BAP, resembling Streptococcus spp.

Answer 212

A

A Both species of Bacillus are catalase and lecithinase positive and produce acid from
glucose. B. cereus is β-hemolytic and motile, but B. anthracis is neither. See the chart
below.
β Hemolysis Motility Oxidase Catalase Lecithinase Glucose
B. cereus + + Neg + + +
B. anthracis Neg Neg Neg + + +

Answer 213

A

D The best specimen is the suspected food itself. Stool cultures are not useful because B.
cereus is part of the normal fecal flora. The suspected food can be the source of food
poisoning by B. cereus if 100,000 or greater organisms per gram of infected food are
demonstrated.

Answer 214

A

A The best differentiating test to perform on a suspected B. anthracis culture is the 10-
unit penicillin disk test. B. anthracis is susceptible, but other Bacillus spp. are not.
Organisms suspected to be B. anthracis should be sent to a reference laboratory for
final confirmation. All tests should be performed in a biological safety cabinet, and
personnel should wear protective clothing to reduce risk from possible production of
aerosols.

Answer 215

A

A The best differentiating test to perform on a suspected B. anthracis culture is the 10-
unit penicillin disk test. B. anthracis is susceptible, but other Bacillus spp. are not.
Organisms suspected to be B. anthracis should be sent to a reference laboratory for
final confirmation. All tests should be performed in a biological safety cabinet, and
personnel should wear protective clothing to reduce risk from possible production of
aerosols.

Catalase Motility CAMP H2S Bile
L. monocytogenes + + (umbrella) Wet prep =
tumbling
+ Neg +
Group B
streptococci
Neg Neg + Neg Neg

Answer 216

A

C E. rhusiopathiae is catalase negative, whereas the other three organisms are catalase
positive. E. rhusiopathiae is seen primarily as a skin infection on the fingers of
workers handling fish, meat, and poultry, as well as in farmers. Colonies growing on
blood agar are small and transparent, may be either smooth or rough, and are often
surrounded by a green tinge. E. rhusiopathiae is characterized by H2S production in
the butt of a TSI slant, which differentiates it from other catalase-negative, grampositive
rods. Motility observation notes a “bottle brush” appearance in gel media at
22°C.

Answer 217

A

A Lactobacillus spp. produce both long, slender rods or short coccobacilli that form
chains. Lactobacillus spp. are part of the normal flora of the vagina (are not considered
pathogens) and are sometimes confused with the streptococci. Lactobacillus spp. are
absent or few when bacterial vaginosis is present with G. vaginalis.

Answer 218

A

D Corynebacterium spp. recovered from a throat culture are usually considered part of
the normal throat flora. C. diphtheriae is an exception and should be suspected when
one of the conditions described occurs. In this event, direct inoculation on Loeffler
serum medium or tellurite medium and the following biochemical tests should be
performed to confirm the identification of C. diphtheriae.
Gelatin hydrolysis = Neg
Motility = +
Acid from glucose = +
Catalase = +
Urease = +
Carbohydrate
fermentation = +

Answer 219

A

D A Gram-stained smear from a vaginal secretion showing many squamous epithelial
cells loaded with pleomorphic gram-variable (positive and negative) bacilli is
considered presumptive for G. vaginalis. Such cells are called clue cells. Other
important findings are:
β-Hemolysis on BAP = +
Oxidase = Neg
Catalase = Neg
Hippurate hydrolysis = +

Answer 220

A

B Nocardia spp. should be suspected if colonies that are partially acid fast by the
traditional method are positive with the modified acid-fast method using Kinyoun stain
and 1% sulfuric acid as the decolorizing agent. The other organisms listed are negative
with acid-fast staining

Answer 221

A

B Serological tests of the patient’s serum for evidence of syphilis are routinely
performed, but culturing is not because research animals must be used for inoculation
of the suspected spirochete. T. pallidum stains poorly or not at all by either the Gram or
acid-fast technique. Darkfield microscopy for direct visualization, phase-contrast, or
indirect immunofluorescence using fluorescein-conjugated antihuman globulin may be
used for visualization of spirochetes. Tests for specific antibodies to T. pallidum are
available in a wide range of immunoassay formats, including chemiluminescence and
point-of-care immunochromatography. Therefore, a reverse algorithm that screens with
a specific test for treponemal antibody is often used. If positive, this is followed by a nontreponemal test (rapid plasma regain [RPR] or Venereal Disease Research
Laboratory [VDRL]) to assess active infection. If the treponemal test is positive but the
nontreponemal test negative, then a second treponemal test, usually the Treponemal
pallidum-particle agglutination (TP-PA), is used to confirm the initial positive
screening test. Also, PCR can be used to identify T. pallidum DNA within lesions and
exudates. Because the bacterium is fastidious, blood samples should be collected in
EDTA, CSF should be frozen and sent on dry ice, and samples should be analyzed as
soon as possible.

Answer 222

A

A Borrelia spp. are often seen on Wright-stained smears of peripheral blood as helical
bacteria with three to 10 loose coils. They are gram negative but stain well with
Giemsa stain. B. recurrentis, is the cause of relapsing fever, a louse-borne epidemic.
The use of thick and thin smears is helpful in observing the organism.

Answer 223

A

D Lyme disease may result in acute arthritis and meningitis and is caused by B.
burgdorferi. This spirochete is carried by the deer tick belonging to the Ixodes genus
(I. dammini in the eastern and north-central United States and I. pacificus in the
northwest United States). The life cycle of the tick involves small rodents, such as the
white-footed mouse and the white-tailed deer.

Answer 224

A

A Serological analysis using immunofluorescence or EIA is the method of choice for
diagnosis of Lyme disease. Titers of IgM remain high throughout the infection. B.
burgdorferi can be cultured directly from lesions, and darkfield microscopy can be
used for detection of spirochetes in blood cultures after 2 to 3 weeks of incubation at
34°C to 37°C. PCR on synovial fluid, CSF, and skin biopsies is used for detection of
early- and late-stage Lyme disease when there is clinical suspicion but serology is
negative

Answer 225

A

B A common cause of respiratory tract illness, M. pneumoniae, generally causes a selflimited
infection (3–10 days) and usually does not require antibiotic therapy. M.
pneumoniae can be cultured from the upper and lower respiratory tracts onto specially
enriched (diphasic) media but is most frequently diagnosed on the basis of the change
in antibody titer from acute to convalescent serum by using EIA or other serological
methods. PCR amplification is used to detect relevant Mycoplasma and Ureaplasma
spp.

Answer 226

A

A Genital mycoplasmas (M. hominis and U. urealyticum) are grown on specific agars.
M. hominis is grown on “M” agar containing arginine and phenol red. Colonies of
mycoplasma are 50 to 300 μm in diameter and display a “fried egg” appearance with
red holes. U. urealyticum is isolated from genital specimens on “U” agar (containing
urea and phenol red) and then subcultured to A7/A8 agar. Colonies of Ureaplasma are
small and golden brown on A7/A8 agar

Answer 227

A

B U. urealyticum is the only human mycoplasma that hydrolyzes urea. The manganous
chloride–urea test utilizes manganous chloride (MnCl2) in the presence of urea. Urease
produced by the organism hydrolyzes the urea to ammonia. This reacts with MnCl2
forming manganese oxide, which is insoluble and forms a dark brown precipitate
around the colonies. The reaction is observed under a dissecting microscope and is a
rapid test for the identification of U. urealyticum.

Answer 228

A

D All of the listed organisms produce mycelium (aerial or substrate), causing them to
appear branched with Gram staining, but only the Nocardia spp. are positive for
modified acid-fast stain. Nocardia is an opportunistic pathogen, and cultures typically
have a “musty basement” odor.

Answer 229

A

A Corynebacterium spp. are part of the normal upper respiratory tract flora. Organisms
display typical pleomorphic shapes often resembling letters, such as “Y” or “L,” and metachromatic granules. Identification of C. diphtheriae, however, requires selective
culture media and biochemical testing.

Answer 230

A

A The “whiff” test is used for a presumptive diagnosis of an infection with G. vaginalis.
A fishy odor is noted after the addition of one drop of 10% KOH to the vaginal
washings. This odor results from the high concentration of amines found in women
with vaginosis caused by G. vaginalis. Presumptive identification includes “clue cells”
seen on Gram-stained vaginal washings or wet-mount preps.

Answer 231

A

B EIA and MIF tests, which are performed on serum, detect antibodies to C.
trachomatis but may also detect antibodies that cross react with other organisms. Cell
culture is time consuming and dependent on the quality of the specimen. PCR testing is
both sensitive and specific and may be performed on urine.

Answer 232

A

A The cold agglutinin test is nonspecific, time consuming, and, therefore, outdated for
the diagnosis of atypical pneumonia. Although M. pneumonia can be grown on
artificial media, many specimens fail to grow, and growth can take up to 3 weeks. CF
tests are not as sensitive or specific as EIA tests. Direct antigen assay by
immunofluorescence is specific but has a sensitivity at least two orders of magnitude
below DNA amplification methods. Although PCR testing is the most sensitive
method, it may detect the presence of M. pneumonia in the absence of infection. EIA
methods are sensitive, but some persons fail to produce antibodies especially in extrarespiratory
infections, and antibody tests alone may detect past, rather than current,
infections. Adults may fail to produce an IgM response, especially in recurrent
infections, so the best confirmatory approach is a combination of EIA and direct
antigen testing

Answer 233

A

A Chlamydophila psittaci is the new taxonomic classification for Chlamydia psittaci.
This bacterium is found naturally in psittacine birds and other avian species.
Psittacosis, the human infection, is the result of contact with pet birds and imported
nonquarantined birds or from occupational contact in poultry farming or processing.
Inhalation of the organisms from aerosols, fecal material, or feather dust causes a
respiratory infection, and the specimen of choice is a throat swab or sputum.

Answer 234

A

D Ehrlichia and Anaplasma spp. are intracellular bacteria and are the causative agents
of human monocytotropic ehrlichiosis (HME) and human granulocytotropic
anaplasmosis (HGA). The organisms infect humans, dogs, and cattle through the bite
of an infected Ixodes tick (named the Lone-Star tick transmitting Ehrlichia spp.),
which also transmits Borrelia spp., causing Lyme disease. Blood, CSF, and serum are
used for serological or PCR testing. A diagnosis can be made by direct observation of
the basophilic inclusions (morulae) in leukocytes of Giemsa- or Wright-stained blood
or buffy coat smears. This occurs in about 20% of HME in which the organisms are
present in the monocytes and more frequently in HGA in which they are found in the
granulocytes

Answer 235

A

B In stages I and II of Lyme disease, EIA is performed on serum for antibodies. PCR
testing and culture of Borrelia spp. may be performed on a skin biopsy but would be
unnecessary in this situation given the characteristic rash associated with Lyme
disease. Synovial fluid, skin biopsy, and CSF may be tested for Borrelia spp. using
PCR when Lyme disease goes untreated and evolves to stage III.

Answer 236

A

A R. equi are facultative intracellular organisms that are found within macrophages. They are found in soil and animal feces. R. equi is usually opportunistic and infects
immunocompromised individuals as well as farm workers through inhalation of the
organisms

Answer 237

A

D Culture techniques are time consuming and difficult to use as a screening procedure.
The fastidious M. genitalium are difficult to grow. Molecular NAAT testing using
conventional PCR will take 2 to 3 days for results, whereas real-time PCR will yield
results in 1 day. M. genitalium has been implicated in the sexual transmission of HIV

Answer 238

A

D In general, anaerobic organisms lack superoxide dismutase (SOD) and catalase
enzymes. The anaerobic bacteria are subdivided on the basis of their O2 requirement.
Obligate anaerobes are killed by exposure to atmospheric O2 for 10 minutes or longer.
Facultative anaerobes grow under aerobic or anaerobic conditions. Microaerophilic
organisms do not grow in an aerobic incubator on solid media and only minimally
under anaerobic conditions. However, they will grow in minimal O2 (5% O2). SOD is produced by many anaerobes and catalyzes the conversion of superoxide radicals to
less toxic H2O2 and molecular O2.

Answer 239

A

C Eh is most affected by pH and is expressed at pH 7.0. In cultivating anaerobic bacteria,
reducing agents, such as thioglycollate and L-cysteine, are added to anaerobic transport
and culture media to maintain a low Eh. Certain anaerobes do not grow in the media
above a specific critical Eh level.

Answer 240

A

A KVA allows growth of Bacteroides spp., Prevotella spp., and Fusobacterium spp. and
inhibits most facultative anaerobic gram-negative rods and gram-positive bacteria
(both aerobic and anaerobic). PEA inhibits facultative gram-negative bacteria but will
support gram-positive aerobes and anaerobes and gram-negative obligate anaerobes.
CCFA is selective for C. difficile from stool, whereas THIO broth supports grampositive
and gram-negative aerobes and anaerobes

Answer 241

A

D The incidence of anaerobic bacteria recovered from the urine is approximately 1% of
isolates. The other three types of infection are associated with a 60% to 93% incidence
of anaerobic recovery. Urine is not cultured routinely under anaerobic conditions
unless the specimen has been obtained surgically (e.g., suprapubic aspiration).

Answer 242

A

A Peptone yeast and chopped meat with carbohydrates support the growth of anaerobic
bacteria. The end products from the metabolism of the peptone and carbohydrates are
volatile fatty acids that help identify the bacteria. After incubation, the broth is
centrifuged, and the supernatant injected into a gas–liquid chromatograph. Peaks for
acetic, butyric, or formic acid, for example, can be identified by comparison with the
elution time of volatile organic acid standards

Answer 243

A

B Anaerobic bacteria can be identified by analysis of metabolic products using GLC.
Results are evaluated, along with Gram staining characteristics, spore formation, and
cellular morphology, to make the identification.

Answer 244

A

C C. perfringens produces a double zone of β-hemolysis on blood agar, which makes
identification relatively easy. The inner zone of complete hemolysis is caused by a θ-
toxin and the outer zone of incomplete hemolysis is caused by an α-toxin (lecithinase
activity). The Bacteroides spp. are gram-negative bacilli, and C. difficile is lecithinase
negative and does not produce a double zone of β-hemolysis

Answer 245

A

A Egg yolk agar (modified McClung or neomycin egg yolk agar) is used to determine the
presence of lecithinase activity, which causes an insoluble, opaque, whitish precipitate
within the agar. Lipase activity is indicated by an iridescent sheen or pearly layer on
the surface of the agar

Answer 246

A

D Lipase is produced by some Clostridium spp. and is seen as an iridescent pearly layer
on the surface of the colonies that extends onto the surface of the egg yolk agar
medium surrounding them. C. perfringens, the most frequently isolated Clostridium
species, is negative for lipase production.

Answer 247

A

B Spores are generally not demonstrated from clinical specimens containing C.
perfringens, which is the only species producing a double zone of hemolysis. The
reactions in the chart above distinguish the four species listed. PCR testing for C.
perfringens identification is available

Answer 248

A

A Spore appearance and location, along with Gram stain morphology, aids in
distinguishing the Clostridium spp. Round, terminal spores (drumstick spores) are
demonstrated when C. tetani is grown in chopped meat with glucose broth.
Recognition of spores is particularly important because C. tetani sometimes appears as
gram negative.

Answer 249

A

A Infant botulism is the most frequent form occurring in the United States.
Epidemiological studies have demonstrated that infant botulism results from the
ingestion of spores via breastfeeding or exposure to honey. Preformed toxin has not been detected in food or liquids taken by the infants. C. botulinum multiplies in the gut
of the infant and produces the neurotoxin in situ.

Answer 250

A

B Foodborne botulism in adults and children is caused by ingestion of the preformed
toxin (botulinum toxins A, B, E, and F) in food. The neurotoxins of C. botulinum are
protoplastic proteins made during the growing phase and released during lysis of the
organisms. C. botulinum is listed as a potential bioterrorism agent (as a possible bioweapon)
by the CDC. Confirmation of botulism is made by demonstration of the
botulinum neurotoxin (BoNT) in food, serum, gastric, or stool specimens. The CDC
can be reached 24 hours a day for emergency assistance when botulism is suspected.
The mouse bioassay is the recommended method for identification of the toxin BoNT,
which is extremely dangerous (Biosafety Level 3).

Answer 251

A

A C. botulinum and C. sporogenes have similar characteristics biochemically (see the
following chart), and definitive identification of C. botulinum is made by the toxin
neutralization test for its neurotoxins in serum or feces. Specimens should be kept at
4°C and sent to the CDC for culture and toxin assays.
Spore Type Motility Lipase GLC
Products
C. botulinum Subterminal + + A, (P)*, B,
(IB)†, IV‡
C. sporogenes Subterminal + + A, (P), B,
(IB), IV

Answer 252

A

B C. difficile is also implicated in hospital- and nursing home–acquired diarrhea and
colitis. Clinical testing for C. difficile includes culture and cytotoxin testing. Because
culture takes 3 days and will detect nontoxigenic strains that do not cause diarrheal
disease, immunoassays using antibodies against either the A toxin or both the A and B
toxins are most frequently employed. Assays detecting both toxins are only slightly
more sensitive because infections producing only B toxin are infrequent. The cytotoxin
assay requires that specimens (only liquid or nonformed stools) be shipped to a
reference laboratory on dry ice or kept at 4°C to 6°C if done in-house for up to 72
hours. Nucleic acid–based assays are also available.

Answer 253

A

D The culture and Gram staining of the puncture wound site usually does not produce
any evidence of C. tetani.The diagnosis is usually based on clinical findings, which are
characterized by spastic muscle contractions, lockjaw, and backward arching of the
back caused by muscle contraction. The tetanus neurotoxin (TeNT) is an exotoxin that
disrupts nerve impulses to the muscles.

Answer 254

A

B The Bacteroides group grows well in 20% bile (primary anaerobic plates include BBE
agar or bile-esculin plates) and is resistant to penicillin two-unit disks with the
exception of B. ureolyticus. Most Prevotella spp. are also resistant to penicillin twounit
disks, but most Fusobacterium spp. and Porphyromonas spp. are sensitive

Answer 255

A

D B. ureolyticus spp. is the only species listed that is susceptible to penicillin and produces urease. The other organisms listed are resistant to penicillin.

Answer 256

A

A Pigmenting Porphyromonas spp. and Prevotella spp. also show hemolysis on
anaerobic blood agar and do not grow on BBE agar

Answer 257

A

B Fusobacterium spp. are usually spindle-shaped, slim rods, whereas the other genera
are small rods (variable length for Bacteroides spp. and tiny coccoid rods for
Prevotella and Porphyromonas spp.). Fusobacterium spp. and Porphyromonas spp. are
susceptible to penicillin two-unit disks, whereas most Bacteroides spp. and Prevotella
spp. are resistant.

Answer 258

A

D Veillonella spp. are gram-negative cocci. All four genera are part of the normal
human flora and are the anaerobic cocci most frequently isolated from blood cultures,
abscesses, wounds, and body fluids. The Streptococcus spp. are facultative anaerobes

Answer 259

A

A P. acnes is a non–spore former and is described as a diphtheroid-shaped rod. It is part
of the normal skin, nasopharynx, genitourinary tract, and gastrointestinal tract flora but
is implicated as an occasional cause of endocarditis and is seen in the inflammatory
process of acne. P. acnes is referred to as the “anaerobic diphtheroid.”

Answer 260

A

D Wounds infected with clostridia are characterized by invasion and liquefactive
necrosis of muscle tissue with gas formation. The most frequent isolate is C.
perfringens followed by C. novyi and C. septicum. C. perfringens is also seen in food
poisoning with the release of the toxin when large numbers of the organism are
ingested

Answer 261

A

A A. israelii is part of the normal flora of the mouth and tonsils but may cause upper or
lower respiratory tract infections. The sulfur granules are granular microcolonies with
a purulent exudate. Like Nocardia, Actinomyces produces unbranched mycelia and is
sometimes (erroneously) considered a fungus. It has also been implicated in pelvic
infection associated with intrauterine contraceptive devices (IUDs).

Answer 262

A

D The anaerobes are not suited for the broth disk elution or disk agar diffusion tests
because of their slow rate of growth. The Kirby-Bauer method reference charts are not
designed to be used as a reference of susceptibility for anaerobes. The three methods
listed use Brucella blood agar or broth, at 35°C to 37°C. incubation (anaerobically) and
take up to 48 hours for results

Answer 263

A

B B. fragilis infections usually occur below the diaphragm and it is the most commonly
encountered anaerobe in anaerobic infections. The Gram-stained smear showing
“safety pin” and pleomorphic bacilli is often seen as a preliminary clue in abdominal
fluids

Answer 264

A

A Contamination by fungi and other bacteria contributes to lower yields of mycobacteria
in a 24-hour sample. The first morning specimen collected by expectoration or
nebulization produces the highest concentration of mycobacteria in sputum.

Answer 265

A

B A strong decontamination solution (6% NaOH or greater) may kill or severely damage
the mycobacteria. A 4% NaOH solution is mixed with an equal volume of N-acetyl-Lcysteine
(NALC), a digestant or mucolytic agent, to yield a final working
concentration of 2% NaOH. The time of exposure of the specimen to the digestion/decontamination solution must be monitored because overtreatment may
result in fewer positive results on culture

Answer 266

A

C All four media contain malachite green as an inhibitory agent of nonmycobacteria, but
the Petragnani medium contains a higher concentration (0.052 g/dL) compared with
the Löwenstein-Jensen (0.025 g/dL), Middlebrook 7H10 (0.0025 g/dL), or ATS (0.02
g/dL) medium. The last is used for normally sterile specimens, such as from CSF and
bone marrow. A combination of solid (17 days for growth) and a liquid (10 days for
growth) media is recommended. If there is no growth after 8 weeks, the cultures are
reported as negative and discarded.

Answer 267

A

A Carbolfuchsin (fuchsin with phenol) stains mycobacteria red and does not decolorize
after the acid–alcohol is added. The background and any other bacterial elements will
decolorize and are counterstained blue by the methylene blue. A fluorescent staining
procedure may be used as an alternative to acid-fast staining. Auramine fluorochrome
produces bright yellow fluorescent mycobacteria and auramine–rhodamine causes an
orange-red (gold) fluorescence against a dark background. A fluorescent microscope
must be used, but with this method, the smear can be scanned with a 25× objective
instead of the 100× objective, permitting more rapid identification of mycobacteria.

Answer 268

A

A M. tuberculosis is positive for niacin accumulation, whereas the other three species are
niacin negative. M. ulcerans is associated with skin infections (in the tropics), does not
grow at 37°C (optimal temperature is 33°C), and is not recovered from sputum. A
serpentine pattern of growth indicates production of cording factor, a virulence factor
for M. tuberculosis.

Answer 269

A

D M. bovis is also called the bovine tubercle bacillus. A nonvirulent strain, bacillus
Calmette-Guérin (BCG), is used as a tuberculosis vaccine throughout the world.
Infections with M. bovis resemble infections caused by M. tuberculosis and are seen in
circumstances where there is close contact between humans and cattle.

Answer 270

A

A Growth rates of mycobacteria are used along with biochemical tests as an aid to
identification. M. fortuitum grows within 3 to 5 days at 37°C and is used as the control
for rapid growers. M. tuberculosis grows in 12 to 25 days at 37°C and is a control
organism for slow growers. In addition to M. fortuitum, M. chelonei is a rapid grower
(3–5 days at 28°C–35°C). In addition to M. tuberculosis, M. avium, and M. kansasii
are slow growers (10–21 days at 37°C).

Answer 271

A

A M. tuberculosis does not produce pigmentation in the dark or after exposure to light
(photochromogen). A common tapwater scotochromogen is M. gordonae. The
pathogenic scotochromogens are M. szulgai, M. scrofulaceum, and M. xenopi. M.
kansasii is a photochromogen producing a yellow pigment following exposure to light
and red β-carotene crystals after long incubation periods

Answer 272

A

D M. kansasii is a photochromogen that causes chronic pulmonary disease (classic
tuberculosis). The other three species cause cutaneous or subcutaneous disease. It is
important to culture skin lesions at the correct temperature to facilitate growth.
Optimal
Temperature
Growth
at 37°C
M. marinum 30°C–32°C Poor
M. haemophilum 28°C–32°C Poor or no
M. ulcerans 33°C No

Answer 273

A

B Niacin production is common to all mycobacteria. However, the niacin accumulates
as a water-soluble metabolite in the culture medium when the organism cannot form
niacin ribonucleotide. M. tuberculosis, M. simiae, and some strains of M. marinum, M.
chelonae, and M. bovis lack the enzyme and therefore are called niacin positive
because of the accumulation of niacin detected in the test medium.

Answer 274

A

D One milliliter of an equal mixture of 30% H2O2 (Superoxol) and Tween 80 (a strong detergent) is added to a 2-week-old subculture on the Löwenstein-Jensen medium and
placed upright for 5 minutes. Catalase activity is determined semiquantitatively by
measuring the height of the column of bubbles produced above the culture surface.

Answer 275

A

A M. bovis and M. tuberculosis are very similar biochemically, and some strains of M.
bovis also accumulate niacin. The T2H test differentiates M. tuberculosis from M.
bovis. M. tuberculosis is not inhibited by T2H.

Answer 276

A

C The hydrolysis of Tween 80 is usually positive when testing the clinically
insignificant mycobacteria. M. fortuitum, M. chelonae, and M. gordonae are
saprophytic (and opportunistic) species, but M. kansasii is a pathogen. M. kansasii
hydrolyses Tween 80 more rapidly than the other species (within 3–6 hours). A
positive reaction is indicated by a change in the color of neutral red from yellow to
pink.

Answer 277

A

A M. xenopi causes a pulmonary infection resembling M. tuberculosis and is frequently
isolated from patients with an underlying disease, such as alcoholism, AIDS, diabetes,
or malignancy. It is often recovered from hot water taps and contaminated water
systems and is a possible source of nosocomial infection. The other three species cause
skin infections and grow on artificial media at a much lower temperature than M.
xenopi (below 32°C).

Answer 278

A

C With the exception of M. tuberculosis, M. avium complex is the Mycobacterium
species most often isolated from patients with AIDS. It is biochemically inert, which is
a distinguishing factor for identification. M. avium complex is highly resistant to the
antibiotics used to treat tuberculosis, including those used in multidrug therapy.
Treatment with streptomycin, rifampin, ethionamide, ethambutol with cycloserine, or
kanamycin has shown little success

Answer 279

A

A Both pathogenic and saprophytic mycobacteria may produce urease, and urease
production is used to differentiate several mycobacteria species. Biochemically, M.
scrofulaceum is identical to M. gordonae, except for the urease reaction for which M.
scrofulaceum is positive and M. gordonae is negative. Urease reactions for the other
pathogenic mycobacteria are:
M. tuberculosis = + M. kansasii = + M. bovis = +
M. avium complex = Neg

Answer 280

A

A The ATS recognizes three levels of laboratory services for mycobacteria testing.
Level I laboratories are those that grow mycobacteria and perform acid-fast stains but
do not identify M. tuberculosis (they may or may not perform drug susceptibility tests
on M. tuberculosis). Level II laboratories perform all of the functions of Level I
laboratories and also identify M. tuberculosis. Level III laboratories identify all
mycobacteria species from clinical specimens and perform drug susceptibility tests on
all species.

Answer 281

A

B The CAP lists four options for laboratories to follow in order to correlate the services
provided with guidelines for inspection and accreditation. A laboratory’s performance
on CAP proficiency tests is evaluated by interlaboratory comparison with laboratories
within these levels of performance.

Answer 282

A

A M. marinum is typically recovered from cutaneous wounds resulting from infection
when the skin is traumatized and comes into contact with inadequately chlorinated
fresh water or salt water, such as in swimming pools or fish aquariums. The other three
species are slow growers at 37°C. M. tuberculosis and M. avium complex are
nonphotochromogens. M. avium complex is urease negative, M. tuberculosis is
positive for niacin and nitrate, and M. kansasii is positive for nitrate and catalase.

Answer 283

A

D M. gordonae is a nonpathogen, scotochromogen, and rapid grower (7 days at 37°C).
Rarely, it is implicated in opportunistic infections in patients with shunts, prosthetic heart values, or hepatoperitoneal disease. The other three species are pathogenic
mycobacteria.

Answer 284

A

D Second-line antibiotics used to treat first-line drug-resistant tuberculosis include
cycloserine, ethionamide, kanamycin, rifabutin, ciprofloxacin, doxycycline, ofloxacin,
SXT and capreomycin

Answer 285

A

D Standard therapy using isoniazid and rifampin for classic, uncomplicated pulmonary
tuberculosis is 9 months. The patient may not respond to therapy, even when the
organism is susceptible to the antibiotics in vitro; therefore, cultures must be kept for
up to 1 year to facilitate testing of additional antibiotics should the infection become
refractory to therapy.

Answer 286

A

A Acid-fast smears are standardized by the ATS for reporting the number of AFB seen.
The following criteria should be used to uniformly report results:
1 to 2 AFB per smear = Report number seen and request another sample
3 to 9 AFB per smear = Report as rare (1+)
10 or more per smear = Report as few (2+)
1 to 9 per oil immersion field = Report as numerous (3+)
Greater than 9 per oil immersion field = Report as (4+)

Answer 287

A

D Mycobacteria spp. growing on MacConkey agar and other routine media are usually
nonpathogens. However, M. chelonae, M. abscessus, and M. fortuitum are rapid
growers that will grow on MacConkey agar (with no crystal violet) within 5 days and
are considered pathogens causing disseminated diseases in immunocompromised
individuals.

Answer 288

A

D M. tuberculosis is a slow grower with a prolonged culture time of 12 to 25 days and
requires 3 to 6 weeks for definitive identification and antibiotic susceptibility testing.
The acid-fast and fluorochrome smears remain the number one rapid test for the initial
detection of mycobacterial infection. A positive smear has a predictive value of 96%
when all laboratory and clinical findings are considered. Mass spectrometry (MALDITOF)
is used for early culture identification when there is moderate culture growth.
Sequencing uses minimal or very little culture growth for identification. DNA probes
are available for rapid identification of M. tuberculosis, M. bovis, and M. avium.
Amplification of a species-specific target region of DNA is done by PCR with a
labeled (biotinylated) oligonucleotide primer. The PCR product is detected by
denaturation and hybridization to a capture probe. After washing to remove unbound
DNA, strepavidin conjugated to an enzyme is added. After washing to remove
unbound conjugate, substrate is added. The presence of product indicates a positive
result.

Answer 289

A

C A positive PPD test result indicates a person who is latently infected with M.
tuberculosis. Such persons are asymptomatic and not infectious but have a 10% risk of
developing tuberculosis during their lifetime.

Answer 290

A

A M. leprae and M. microti are different from all other mycobacteria because they
cannot be cultured in vitro. Biopsies from nodules and plaques of skin that show
numerous AFB are presumptively diagnosed as positive for M. leprae. Confirmatory
identification is made by nucleic acid testing (NAT) using PCR.

Answer 291

A

A Greater than 90% of M. avium complex organisms that are isolated from patients with
AIDS are M. avium. These are differentiated from M. intracellulare and M. chimaera,
which comprise the other two members of the complex by performing NAT.

Answer 292

A

B M. microti is found in guinea pigs, rabbits, cats, and other warm-blooded animals. It is
a cause of tuberculosis in immunocompromised humans but is known to cause
tuberculosis in immunocompetent persons as well. M. microti cannot be cultured in
vitro but can be distinguished from M. leprae by its distinctive “croissant-like”
morphology. Diagnosis is confirmed by performing NAT.

Answer 293

A

A M. haemophilum, M. ulcerans, and M. marinum grow optimally at 27°C to 30°C. M.
xenopi grows optimally at 42°C to 45°C, and is frequently isolated from hot water
systems and storage tanks. All other mycobacteria are cultured at 35°C to 37°C.

Answer 294

A

B M. avium subsp. paratuberculosis is known to cause an ulcerative intestinal disease
with chronic diarrhea in cattle known as Johne disease. Although M. paratuberculosis
has been isolated from the intestines of humans with Crohn disease with significantly
greater frequency than controls, the organism has not yet been proven to cause ileitis in
humans

Answer 295

A

B M. marinum is usually recovered from skin lesions that have been in contact with
underchlorinated freshwater, saltwater, or tropical fish aquariums

Answer 296

A

A M. tuberculosis tests positive in the niacin (nicotinic acid) accumulation test, which is
done after sufficient cultural growth is observed. Even though all other species produce
niacin, M. tuberculosis produces the most which is tested directly on the L-J slant that
is at least 3 weeks old, provided sufficient numbers of the typical buff-colored, slowgrowing,
rough colonies (serpentine) are observed.

Answer 297

A

D This smear cannot be reported as positive or negative because only two AFB were
observed per 300 fields. The two AFB observed could be contaminants in the oil
immersion or on the lens of the microscope from a previous positive AFB smear. A
new specimen should be requested. The fluorochrome staining method saves time
when scanning smears for AFB. With the use of a UV microscope, a lower
magnification (450×) allows for more fields to be scanned in less time. Positive results
using a fluorochrome staining method, must be confirmed using an acid-fast staining
method.

Answer 298

A

B Presently, BCG (an attenuated strain of M. bovis) has been used in many parts of the
world to vaccinate susceptible individuals against tuberculosis.

Answer 299

A

D Specimens for fungal culture must be kept in a moist, sterile environment. Swabs that
are dried out or submitted with insufficient material on them should be rejected.
Generally, swabs are inadequate for the recovery of fungi because they are easily
contaminated by surrounding skin flora.

Answer 300

A

A A solution of 10% KOH is used for contaminated specimens, such as skin, nail
scrapings, hair, and sputum, to clear away background debris that may resemble fungal
elements. Normally sterile specimens (CSF, blood, and bone marrow) do not require
KOH for clearing

Answer 301

A

B Meningitis caused by C. neoformans is diagnosed through culture, biochemical
reactions, and rapid agglutination tests for cryptococcal antigen. The India ink test is
not diagnostic for cryptococcal meningitis because positive staining results are
demonstrated in less than 50% of confirmed cases. A positive India ink test shows
yeast cells in CSF with a surrounding clear area (the capsule) because the capsule of C.
neoformans is not penetrated by ink particles

Answer 302

A

B Superficial dermatophytes rarely invade the deeper tissues and are the cause of most
cutaneous fungal infections. Fungal infections of the skin are most often caused by
Microsporum spp., Trichophyton spp., and Epidermophyton spp., although Candida
spp. are sometimes implicated as the cause of nail infections.

Answer 303

A

D The true germ tube (filamentous extension from a yeast cell) is approximately one-half
the width and three to four times the length of the cell with no true hyphae constriction
at the point of origin. C. albicans produce germ tubes (95%), and a positive test is
considered a presumptive identification

Answer 304

A

A C. tropicalis forms pseudohyphae that resemble true germ tubes by producing a
constriction at the point of origin of the yeast cell. Germ tubes represent true hyphae
without constriction, and therefore the test should have been repeated along with
carbohydrate tests before making a presumptive identification. The other three species
of yeast listed do not form hyphae

Answer 305

A

D Cornmeal agar with Tween 80 (polysorbate) reduces the surface tension and allows for
enhanced formation of hyphae, blastospores, and chlamydoconidia

Answer 306

A

C Pseudohyphae are the result of a pinching-off process, blastoconidiation, with the
growth of filaments with constrictions. Germ tubes are the beginning of true hyphae
(no constrictions). Arthrospores are the result of a breaking-off process of true septate
hyphae resulting in square conidia.

Answer 307

A

A A yeast that is isolated from CSF and is producing blastospores is most likely to be C.
neoformans, which is positive for urease and produces brown colonies on birdseed
agar.

Answer 308

A

A Most isolates of C. neoformans produce phenol oxidase when grown on Guizotia
abyssinica medium (birdseed medium), producing brown to black pigmented colonies.
C. neoformans is the only Cryptococcus species that oxidizes o-diphenol to melanin,
which is responsible for the color.

Answer 309

A

D C. albicans and Candida dubliniensis, a variant of C. albicans, are the only yeasts
that produce germ tubes within 1 to 3 hours of incubation at 37°C. C. tropicalis
produces pseudohyphae after incubation for 3 hours, which may be mistaken for germ
tubes. A careful evaluation of the tube origin for constriction is required to avoid a
false-positive interpretation

Answer 310

A

B T. cutaneum and C. neoformans are both urease positive, but T. pullulans produces
arthroconidia, and C. neoformans does not. In addition to Trichosporon spp.,
arthroconidia are produced by Geotrichum spp.

Answer 311

A

B Germ tube–negative isolates producing dark brown to black colonies on niger seed
agar and a positive urease test are presumptive of C. neoformans. A positive germ tube
test result is a presumptive identification for C. albicans as well as for C. dubliniensis.
See the chart above.

Answer 312

A

D Cryptococcus spp. do not form either pseudohyphae or arthroconidia. Candida spp.
produce blastoconidia or pseudohyphae. Trichosporon spp. produce pseudohyphae,
blastoconidia, and arthroconidia. See the chart below

Answer 313

A

A Sexual spore production is a characteristic of the Ascomycotina, which produce an
ascus (saclike structure) after the union of two nuclei. The resulting spore is termed
ascospore. S. cerevisiae produces ascospores when grown on ascospore agar for 10
days at 25°C.

Answer 314

A

B Rhodotorula spp. produce pink- to coral-colored colonies on Sabouraud agar and
cornmeal agar. It is usually considered a contaminant but is an opportunistic pathogen,
and must be identified when found in specimens from immunosuppressed patients.

Answer 315

A

C Cornmeal Tween 80 agar supports the growth of C. albicans and the formation of its
distinctive thick-walled, usually single, terminal chlamydoconidia. C. dubliniensis also
produces chlamydoconidia (in pairs, triplets, and clusters) and germ tubes but is rarely
implicated in disease

Answer 316

A

A The yeast isolate is inoculated directly into the molten agar base free of carbohydrates
or is poured as a suspension onto a yeast nitrogen agar base plate. Carbohydrate disks
are then added to the surface of the agar, and the plates are incubated for 24 to 48
hours at 30°C. Growth around the disk indicates the ability of the yeast to utilize the
carbohydrate(s) as a sole source of carbon.

Answer 317

A

D This isolate is C. albicans, which also produces some true hyphae along with
pseudohyphae. A positive germ tube is a presumptive identification along with the
production of blastoconidia, terminal chlamydoconidia, and pseudohyphae

Answer 318

A

A C. tropicalis and C. kefyr (pseudotropicalis) differ in their arrangement of
blastoconidia along the pseudohyphae. C. kefyr (pseudotropicalis) forms elongated
blastoconidia arranged in parallel clusters that simulate logs in a stream. Trichosporon
spp. and Geotrichum spp. form arthroconidia

Answer 319

A

B Dimorphic molds are in the yeast form in infected tissues because they are in the yeast
form at 37°C. Specimens are cultured and incubated at both room temperature and
35°C to 37°C. To prove that a mold growing at room temperature (or 30°C) is a
dimorphic fungus, conversion to the yeast form must be demonstrated via subculture
and incubation at 37°C.

Answer 320

A

A The mold form of C. immitis shows barrel-shaped arthroconidia separated by empty
cells (ghost cells) that cause an uneven staining effect when they are examined under a
microscope. S. schenckii, H. capsulatum, and B. dermatitidis produce conidia that are round or oval in shape in the mold phase.

Answer 321

A

B S. schenckii is usually acquired by humans through thorns or splinters because it is
commonly found on living or dead vegetation. It is called “rose gardener’s disease”
because gardeners, florists, and farmers are most often infected. S. schenckii is often
recovered from exudates of unopened subcutaneous nodules or open draining lesions.

Answer 322

A

A Sepedonium spp. are saprophytic molds that do not have a yeast phase and produce
large spherical tuberculate macroconidia, similar to H. capsulatum. Histoplasmosis is a
chronic granulomatous infection primarily found in the lungs that invades the
reticuloendothelial system. Infection occurs via spores released from decaying bird or
chicken droppings that are inhaled when disturbed.

Answer 323

A

A P. brasiliensis yeast forms are sometimes seen as a “mariner’s wheel” because
multiple budding cells completely surround the periphery of the parent cell.

Answer 324

A

B Mucorales (formerly Zygomycetes) commonly recovered from clinical specimens are
Rhizopus spp. and Mucor spp. Both display aseptate hyphae, whereas the other groups
above display septate hyphae. Mucorales (Zygomycetes) usually not encountered in
clinical specimens are also aseptate and include Lichtheimia spp. (formerly Absidia
spp.), Rhizomucor spp., Cunninghamella spp., and Syncephalastrum spp.

Answer 325

A

A M. furfur has a worldwide distribution and causes a superficial, brownish, dry, scaly
patch on the skin of light-skinned persons and lighter patches on persons with dark
skin. M. furfur is not cultured because diagnosis can be made from microscopic
examination of the skin scales. Skin scrapings prepared in KOH show oval- or bottleshaped
cells that exhibit monopolar budding in the presence of a cell wall and also
produce small hyphae

Answer 326

A

D Trichophyton spp., Microsporum spp., and Epidermophyton spp. are the organisms
causing human dermatomycoses or cutaneous infections. Trichophyton spp. infect hair
and nails, as well as skin. Infections with members of the genus Microsporum are
confined to the hair and skin, whereas infections caused by the genus Epidermophyton
are seen only on skin and nail

Answer 327

A

A Members of the genus Microsporum produce club-shaped microconidia and are
usually pigmented white, buff, yellow, or brown. Epidermophyton does not display
microconidia and produces yellow-green or yellow-tan colonies. T. rubrum can be
differentiated from the other members of the genus by its distinctive cherry-red color.
Trichophyton mentagrophytes may also produce a red pigment, but it is usually rose
colored or orange, or deep red. T. tonsurans produces white-tan to yellow, suedelike
colonies. T. schoenleinii produces white to cream-colored colonies, and T. violaceum
produces port wine to deep violet colonies.

Answer 328

A

B M. audouinii and T. tonsurans may both cause epidemic tinea capitis in children. M.
audouinii causes a chronic infection transmitted directly via infected hairs on caps,
hats, combs, upholstery, and hair clippers. Infected hair shafts fluoresce yellow-green
under a Wood lamp. M. audouinii does not usually sporulate in culture and forms
atypical vegetative forms, such as antler and racquet hyphae and terminal
chlamydoconidia. In contrast, M. canis produces spindle-shaped, thick-walled
multicelled macroconidia, and M. gypseum produces ellipsoidal, multicellular
macroconidia.

Answer 329

A

C Epidermophyton spp. do not produce microconidia; this differentiates them from
Trichophyton spp. and Microsporum spp. Alternaria is not a dermatophyte.
Epidermophyton floccosum is the most frequently isolated member of the genus and
infects skin, but not hair

Answer 330

A

D T. schoenleinii is identified microscopically by its characteristic antler-shaped hyphae
and chlamydoconidia in the absence of conidia.

Answer 331

A

A T. mentagrophytes may produce a deep red pigment seen through the reverse side of
the agar plate that resembles the cherry-red pigment produced by T. rubrum. However,
T. mentagrophytes can be differentiated by its ability to invade the hair shaft. T.
rubrum grows on the surface of hair but does not penetrate the shaft.

Answer 332

A

A The dematiaceous molds are easily recognized and confirmed by observing darkyellow
or brown septate hyphae upon microscopic examination

Answer 333

A

B A. niger is the only species listed producing black conidia, which causes a “pepper”
effect as the colony grows. The reverse side of the agar plate remains buff or cream
colored, which differentiates it from the dematiaceous (dark) molds.

Answer 334

A

A Phialophora, Exophiala, and Wangiella all produce phialides, but the last two genera
form elongated, tubelike phialides without a collarette, as opposed to the flask-shaped
phialides of Phialophora, which contain clusters of conidia at the tips.

Answer 335

A

C A. fumigatus is most often associated with compost piles and is found in the soil of
potted plants. A. niger is the cause of cavitary fungus ball lesions of the lungs and nasal
passages.

Answer 336

A

A Fusarium spp. are usually a contaminant but are sometimes seen as a cause of
mycotic eye, nail, or skin infection in debilitated patients. Fusarium is a hyaline (light)
mold and grows on Sabouraud agar plates at 30°C within 4 days. The other three
organisms are members of the Dematiaceae family (dark molds).

Answer 337

A

B A. fumigatus is the most common cause of aspergillosis. It is characterized by
sporulation only from the upper half or two-thirds of the vesicle. Colonies of A. niger
are white with black pepper growth and produce phialides over the entire vesicle,
forming the classic “radiate” head. A. flavus colonies are yellow to yellow-green and
produce phialides that cover the entire vesicle and point out in all directions. A. terreus
produces brown colonies and phialides that also cover the entire vesicle.

Answer 338

A

A Lichtheimia (Absidia) spp. are similar to Rhizopus spp. except for the location of
rhizoids (rootlike hyphae). The rhizoids of Rhizopus spp. are located at the point where
the stolons and sporangiophores meet, whereas those of Lichtheimia (Absidia) spp.
arise at a point on the stolon between the rhizoids. Penicillium spp. and Aspergillus
spp. do not form rhizoids.

Answer 339

A

B Immunocompromised patients are at risk for invasion of C. neoformans. The
polysaccharide capsule of C. neoformans is not recognized by phagocytes, which cause
patients with impaired cell-mediated immunity to become readily infected with C.
neoformans

Answer 340

A

A Thermally dimorphic H. capsulatum produce microconidia and hyphal fragments at
37°C (yeast phase), whereas at 25°C (mold phase), the organism displays large, thickwalled,
round macroconidia with knobby or knoblike projections. The yeast form is
able to survive within circulating monocytes or tissue macrophages that can be
demonstrated with Giemsa or Wright stain.

Answer 341

A

D C. immitis endospores are often confused with yeast cells, but they do not bud. C.
immitis is endemic in the southwestern United States. Because arthroconidia are highly
infectious, an open plate should not be used, and a slide culture test should not be
performed. Rather, tubed media is used for testing, and all work should be performed
in a biological safety cabinet.

Answer 342

A

A P. jirovecii (P. carinii) is classified as a fungus but formerly was classified as a
parasite. It is best recovered by bronchoalveolar lavage or induced sputum in
immunocompromised patients. Open lung biopsy sample was the specimen of choice
before the AIDS epidemic. GMS stain is used to identify the organism which stains the
cyst form in lung tissue, but not the trophic form. By using hematoxylin and eosin
(H&E) staining on a smear, an eosinophilic, foamy material can be detected which is
formed from the successful replication of the organism in alveolar lining fluid.

Answer 343

A

B Other Penicillium spp. are differentiated from Talaromyces (Penicillium) marneffei
(thermally dimorphic) through conversion from the mold to yeast phase. Talaromyces
(P.) marnerffei are seen as yeast at 35°C to 37°C on 5% sheep blood agar or in BHI
broth. Other Penicillium spp. do not display a yeast phase. T. marneffei are recovered
from the blood, skin, lymph nodes, bone marrow, and internal organs of
immunocompromised patients.

Answer 344

A

A Open lung biopsy is not recommended for persons with AIDS because of the
increased risk of death, infection, and complications associated with this surgical procedure. Persons with AIDS and P. jirovecii (carinii) pneumonia (PCP) can be
diagnosed with chest radiography, special stains, and PCR methods from specimens
obtained by bronchoalveolar lavage (BAL). Patients without AIDS have a much lower
yield, and therefore, besides BAL, tracheal aspiration, endoscopic lung biopsy, cellular
material from bronchial brushings, and open lung biopsy specimens are more often
indicated.

Answer 345

A

C To determine an early diagnosis, soluble antigens (the galactomannan assay targets
antigens of Aspergillus spp.) are detected by using such methods as ELISA, latex
agglutination, and immunoblotting using serum or body fluids. Blood and sputum
cultures are usually negative, and tissue biopsy is associated with a significantly higher
risk of complications. Nucleic acid amplification assays, multiplex amplification and
reverse transcription PCR (RT-PCR) have been developed which detect systemic
aspergillosis in respiratory specimens. MALDI-TOF MS methodology has proven
successful on old and young cultures, with 98.6% correct identification.

Answer 346

A

D Vascular invasion, followed by thrombosis, tissue infarction, and necrosis are the
most common clinical manifestations of cutaneous infections caused by Rhizopus spp.
Infections occur mainly in patients who are already being treated for other conditions,
including lymphoma, leukemia, neutropenia, renal failure, and diabetic ketoacidosis.

Answer 347

A

A The yeast phase of H. capsulatum develops as small oval budding cells seen in
macrophages. The yeast phase of P. brasiliensis develops large cells that bud creating
a structure resembling a mariner’s wheel, and C. immitis produces very large
spherules. At 25°C, P. brasiliensis produces small single conidia, and C. immitis produces arthroconidia. Candida spp. are not dimorphic. H. capsulatum is found in soil
containing excrement of birds and bats. It is endemic in the Ohio and Mississippi River
valleys, and the most common systemic mycosis in North America.

Answer 348

A

A C. immitis is a soil fungus and is endemic to the southwestern United States. Infection
results from inhaling the arthroconidia, which form endospore-forming spherules and
leads to infection within 1 to 3 weeks. Nonimmunocompromised people usually do not
require treatment and are immune to reinfection. However, immunodeficient persons
may develop symptomatic pulmonary and multiorgan infections

Answer 349

A

A M. furfur, the cause of tinea versicolor, is often diagnosed from a direct skin scraping
specimen. The classic “spaghetti and meatball” formations of the yeast and hyphal
cells are distinct. In fungemia, the blood culture specimen will show small yeast cells
without pseudohyphae

Answer 350

A

B Microscopically, Scopulariopsis spp. resemble Penicillium spp. except for the following: Scopulariopsis spp. produce annellophores with flask-shaped annelides
bearing large lemon-shaped, rough-walled conidia in chains; Penicillium spp. display
brush-like conidiophores producing metulae with flask-shaped phialides, resulting in
chains of small, smooth-walled conidia

Answer 351

A

C A “foot cell” is characteristic of Aspergillus fumigatus. The presence of a foot cell,
which is a T- or L-shaped cell at the base of the conidiophore (it is not a separate cell),
is characteristic and distinct for A. fumigatus which is thermotolerant, growing at
temperatures up to 45°C.

Answer 352

A

C True viruses have nucleic acid that is either RNA or DNA, and this serves as the basis
for initial classification. Members of these classes are further divided into groups that cause human disease on the basis of mode of transmission, tissues invaded, diseases
produced, and antigenic characteristics

Answer 353

A

A RSV is the cause of croup, bronchitis, bronchiolitis, and interstitial pneumonia.
Hospitalized children age less than 1 year are the most susceptible group.

Answer 354

A

D Coxsackie A virus, Coxsackie B virus, and the echoviruses are most commonly
implicated in myocarditis and other syndromes, including acute cerebellar ataxia and
hepatitis. Like poliovirus, infections are more common in the summer and fall seasons
and gain entry through the gastrointestinal tract

Answer 355

A

A CMV infection in a previously healthy individual causes a self-limiting mononucleosis
syndrome. CMV is an opportunistic pathogen that may produce lifelong infections and
causes a variety of diseases, including congenital and neonatal infection, hepatitis,
pneumonia, and disseminated infection in immunocompromised patients

Answer 356

A

A Influenza and adenoviruses are the main causes of respiratory infections, including the
common cold, tracheobronchitis, and pneumonia. Adenoviruses also cause
conjunctivitis, keratitis, cystitis, and gastroenteritis and are transmitted by aerosolized
droplet, contaminated respiratory secretions, stool or fomites

Answer 357

A

B Rotaviruses have been implicated in both nosocomial infections and epidemic
gastroenteritis. Children ages 3 to 24 months are most commonly affected. Diarrhea
begins after an incubation period of 3 days, lasts for 2 to 10 days and is associated with
vomiting and dehydration. In immunosuppressed children, rotavirus causes a chronic
infection.

Answer 358

A

D Parvovirus causes a fever and characteristic “slapped cheek” rash in young children.
Adults are usually immune, but immunocompromised persons may exhibit arthritis or
anemia (the virus infects immature RBCs in bone marrow).

Answer 359

A

D Although the most common mode of transmission of hepatitis B is via needle puncture,
it may also be transmitted by other parenteral means, including through sexual
transmission and contact with contaminated blood via broken skin or mucous
membranes

Answer 360

A

A Noroviruses are small RNA viruses that have been implicated in epidemics of
community gastroenteritis as well as sporadic infections. Unlike rotaviruses, which
cause gastroenteritis in infants and young children, noroviruses produce infections in
all age groups

Answer 361

A

D Human papillomaviruses (HPVs) cause genital warts, the most prevalent sexually
transmitted viral disease in the United States. Several strains, including HPV-6, HPV-
11, HPV-16, and HPV-18, are associated with penile, cervical, and vaginal neoplasia,
causing more than 60% of the cancers. Because the virus cannot be cultured in vitro,
diagnosis is usually made by using DNA probes, histopathology, or cytological
examination of cutaneous biopsy of cells. A diagnostic characteristic of infected cells
is koilocytosis, a perinuclear clearing in the squamous epithelium accompanied by
nuclear atypia. Currently, the FDA has licensed three vaccines for HPV: Cervarix,
Gardasil, and Gardasil-9.

Answer 362

A

D In addition to serological tests for antibodies against the virus and DNA probes that
identify viral DNA or RNA, cytological and histological examination aid in the rapid diagnosis of several viruses. The rubella virus is detected serologically by testing for
specific IgG and IgM antibodies. Testing is routinely done on pregnant women, as a
prenatal precaution.

Answer 363

A

C The Western blot assay is no longer used to confirm a positive serological test of
antibodies to HIV. Western blot detects antibodies but it may miss people in the
window phase of infection, is not confirmatory for neonatal infections because of the
presence of maternal antibodies, and has a long turnaround time resulting in loss of
contact with the patient. For these reasons, the CDC recommends a new approach
using a simultaneous test for antibodies to both HIV-1 and HIV-2 and p24 antigen to
screen for infection by immunoassay. Positive specimens would be tested by separate
immunoassays to identify whether the infection is HIV-1 or HIV-2. Samples testing
negative in suspected cases should be tested by a nucleic acid amplification method,
such as PCR for viral RNA for definite identification.

Answer 364

A

C Using direct immunofluorescence microscopy (DIF), rabies antigen can be detected in
the cutaneous nerves surrounding the hair follicles of the posterior region of the neck
(nuchal biopsy) and in the epithelial cells obtained through corneal impression.
Antibodies to rabies can be detected in the serum and CSF of infected persons within 8
to 10 days of illness; however, infection usually occurs several months before the onset of symptoms. Isolation of virus from the saliva of the patient may be accomplished by
mouse inoculation or by inoculation of susceptible cell culture lines with subsequent
detection by immunofluorescent antibodies. Postmortem diagnosis of rabies is
accomplished by using direct immunofluorescent assay of brain tissue. Brain tissue
testing from the dead animal results in prompt treatment/prophylaxis of the human
victim after an animal bite or injury

Answer 365

A

D Influenza virus types A, B, and C may be grown and isolated in cell cultures by using
primary monkey kidney (PMK) cells permitting identification within 1 to 3 days in
specialized laboratories performing viral cultures. Direct fluorescent and enzyme
immunoassays by using monoclonal antibodies to nucleoprotein antigens in infected
nasal epithelium are used for rapid diagnosis of both influenza A and influenza B
infections. Influenza C testing is not routinely requested, and this type is referred to as
“mild” influenza. RT-PCR is replacing the cell culture method because of better turnaround
time, from collection of specimen to identification. It is also becoming the
gold-standard for identifying respiratory viruses.

Answer 366

A

A DIF testing of vesicle (lesion) fluid for virus using fluorescein-conjugated antibodies
is the most rapid method for diagnosis of genital herpes infection. Immunofluorescence
and immunoperoxidase methods are also used to distinguish HSV-1 and HSV-2. PCR
and real-time PCR are more sensitive rapid methods for detection and identification.
Viral cell culture is also very sensitive and may yield a positive result within 24 hours
when fluid contains a high concentration of virus. Vero cells or primary human
embryonic cells are inoculated with vesicle fluid and examined for cytopathic effects
(CPEs), the most common of which are large “balloon” cells and multinucleated giant
cells.

Answer 367

A

C EBV serological testing for IgM-VCA (viral capsid antigen) during the illness may or
may not be positive. Patients who present with an infectious mononucleosis–like
syndrome should be tested for both EBV and CMV. Both viruses cause the same
symptoms during the acute phase of the illness

Answer 368

A

A West Nile virus causes neurological diseases with meningitis and encephalitis at the
top of the list. The animal reservoirs are birds, with humans being accidental hosts.
Transmission of West Nile virus is from mosquito to bird.

Answer 369

A

A Hantavirus is transmitted by a rodent host, the deer mouse, was originally endemic in
the southwest United States, but currently has been discovered throughout the country.
The name of the hantavirus responsible for outbreaks in the southwest is the Sin
Nombre virus. Breathing in excrement of mice is the most common mode of infection,
and the lung is the site of initial infection, causing labored breathing or hantavirus
pulmonary syndrome (HPS). Diagnosis is usually made by using an IgM/IgG ELISA
assay and RT-PCR. Isolation of the virus by culture has not been accomplished to date.
The primary site of infection for norovirus and rotavirus is the gastrointestinal area and
for hantavirus the pulmonary sector.

Answer 370

A

D Rotavirus is one of the most common causes of gastroenteritis in infants and young
children (ages 6 months to 2 years). Vomiting and diarrhea are also common
symptoms of norovirus infections, but the prevalence of rotavirus during the winter
months and the absence of illness in other family members point to rotavirus being a
more likely cause. Commercial availability of immunoassays for rotavirus makes its
diagnosis easier to establish and rule out than infection with norovirus.

Answer 371

A

D The decision to initiate antiviral therapy is based on the presence or absence of
symptoms, CD4 lymphocyte count, and the viral load. For example, treatment is
usually withheld from patients with CD4 counts greater than 350/μL and viral load less
than 55,000/mL and is instituted in asymptomatic patients if the CD4 count is less than
200/μL, regardless of viral load. Treatment failure within the first year with three-drug
regimens is 35% to 45%, and drug resistance testing (genotype and/or phenotype testing) is recommended to identify drug-resistant strains prior to initiating treatment.
For successful antiviral therapy, the HIV plasma viral load should be reduced to
undetectable levels.

Answer 372

A

A RSV is spread by large particles, such as dust, and is one of the most common causes
of hospitalization for respiratory illness in infants age less than 1 year. RSV causes
bronchiolitis, pneumonia, and croup in infants and upper respiratory illness in children.
It has also been found to cause nosocomial infection in nursing homes. Diagnosis is
made by using EIA, fluorescent antibody (FA) staining, and cell culture (HEp-2).

Answer 373

A

B Rabies virus can be detected by FA staining and PCR testing. The virus replicates at
the site of the bite and penetrates the surrounding tissue, finding its way to the central
nervous system. Because the source cannot be tested, the best immediate course of
action is to initiate postexposure prophylaxis with antirabies globulin and to immunize
the patient with the rabies vaccine.

Answer 374

A

A SARS virus was first identified in China in November 2002 when it caused a
worldwide outbreak that was finally contained by July 2003. The virus belongs to the
group of coronaviruses causing the common cold and is easily transmitted to health
care workers who have close contact with infected individuals. This virus is the cause
of a severe lower respiratory infection that can be fatal. Laboratory confirmation may
be done by using the recommended method, that is, RT-PCR testing, which is available
commercially. Typically, PCR is used on two different specimen types or the same
specimen type submitted at least 2 days apart. If both tests are positive, the infection is
confirmed. Cell culture (Vero-E6 cell line) can be performed if done in a BSL-3 or
higher-level laboratory.

Answer 375

A

D HSV-2 infections produce genital lesions. Infants born prematurely with disseminated
infection of HSV-2 from HSV-positive mothers have a mortality rate of 50% to 60%.
Testing of pregnant women for antibody and cesarean section delivery can prevent
most neonatal HSV infections because the virus enters the fetus during the delivery
process

Answer 376

A

B Varicella-zoster virus is the cause of an infection with chickenpox. As an adult, the
father is experiencing shingles, a reactivation of the virus. The virus lies dormant in the
sensory (dorsal root) ganglia of the spinal nerves, and its reactivation produces a
nonweeping, blisterlike rash on an inflamed skin base that follows the path of the
underlying nerves.

Answer 377

A

B Transmission of rabies (rhabdovirus) occurs primarily from the bite of a rabid animal,
but in 20% of human rabies cases, there is no known exposure to rabid animals. Donor
tissues are not routinely tested for rabies. Diagnosis is achieved by performing
fluorescent antibody staining of the infected tissues and electron microscopy, which
shows the presence of bullet-shaped virus particles.

Answer 378

A

D Adult gastroenteritis caused by norovirus follows a 1 to 2 day incubation. Diagnosis
is usually made on the basis of clinical criteria and confirmed by EIA, RT-PCR, or
immune electron microscopy. Hepatitis E virus, also of the Caliciviridae family, also
causes gastroenteritis 1 to 2 days after exposure but is not endemic in the United States
and other developed countries

Answer 379

A

B Western blot is a test for detecting antibodies to HIV, and sufficient antibodies are not
usually present in the first 3 to 4 weeks of infection to cause a positive test result. In
some persons, antibodies do not appear until 3 months after the initial HIV infection.
RT-PCR detects viral RNA and reduces the window phase of infection to
approximately 1 week. RT-PCR tests for HIV-1 have 100% specificity at a cutoff of 20
to 40 copies RNA/mL. PCR is the preferred test for neonates because it avoids
detection of maternal antibody and has replaced the Western blot as the confirmatory
test of choice.

Answer 380

A

B Tests for toxoplasmosis, German measles, CMV, and HSV comprise a panel of
infectious agents known as the TORCH panel. The TORCH panel consists of
serological tests for antibodies to these organisms and has largely been replaced by more sensitive NAT when infection is suspected. The “O” in TORCH stands for
“other,” and other viruses that cause transplacental infection, such as HBV and HIV
are usually tested prenatally

Answer 381

A

A BK virus, a polyoma virus, is transmitted by direct contact with infected respiratory
secretions. It has tropism for the urinary system and often causes a latent,
asymptomatic infection in the kidneys. However, in immunocompromised patients, the
virus is often implicated in renal and bladder infections. Kidney failure caused by BK
virus is a significant concern in bone marrow and renal transplant recipients.

Answer 382

A

A The Ebola RNA virus is endemic in the Democratic Republic of the Congo with a
human case fatality rate of 80% to 90%. The virus is transmitted from person to person
or to caregivers through body fluids and respiratory droplets. Transmission of the virus
to humans is from monkeys or other wild animals. It is speculated that bats harbor the
virus as carriers or reservoirs. Using electron microscopy, the shape described is
similar to that of the Marburg virus (“shepherd’s hook” shape), which causes a less
severe viral hemorrhagic fever with a fatality rate of 23% to 25%. For diagnosis,
electron microscopy (research laboratories), RT-PCR, and antibody detection with
ELISA (IgG and IgM) are used along with cell culture in monkey cells in a BSL-4
laboratory. Both infections have no cure or established drug treatment. Several Ebola
vaccines are being tested in clinical trials, as reported by the World Health
Organization (WHO).

Answer 383

A

C The Zika virus is transmitted by the bite of the Aegypti mosquito, and the virus
disappears from blood after 2 weeks; however, the virus can linger in sperm for 2
months. Zika virus outbreaks were seen in Brazil and southern Mexico. According to
the Pan-American Health Organization (PAHO), reports of local transmission of Zika
virus occurred throughout Mexico, with a peak in number of infections occurring in
October 2016. To curb the transmission of the virus, genetically modified mosquitoes
were used; after mating, the offspring would die before reaching adulthood. Spraying
with insecticides kills the adults and larvae, and this was one way used to control
outbreaks that occurred in Miami in 2016.

Answer 384

A

A SARS virus is detected with RT-PCR and can be cultured by using the Viro-E6 cell
line. Low levels of the virus early in the infection poses a problem in diagnosing
SARS.

Answer 385

A

B Hepatitis E virus (HEV) infection is the result of an infection with a new genus,
Hepevirus. It is transmitted by the fecal–oral route and mimics hepatitis A, except that
HEV has a high rate of fatality (10%–20%) among pregnant women. It is not endemic
in the United States or in other developed areas of the world. Pregnant women should
be cautioned about drinking contaminated water and traveling to parts of the world
where HEV is endemic (India and Pakistan). Fulminant hepatitis, with a fatality rate of
30%, results if infection occurs during the third trimester. Diagnosis is through
serology (IgG and IgM antibodies) as well as liver function tests (aspartate
aminotransferase [AST], alanine aminotransferase [ALT], and bilirubin) during the
early phase of the infection

Answer 386

A

C The appropriate procedure for the diagnosis of E. vermicularis would involve the
microscopic examination of a series of consecutive cellulose tapes. Typical eggs and/or
a rare adult pinworm could be identified under the microscope.

Answer 387

A

B As PMNs in stool begin to fragment and appear to have four nuclei, they will resemble
E. histolytica/E. dispar cysts. However, E. histolytica/E. dispar cysts are rarely seen in
cases of diarrhea. The species name E. histolytica is reserved for the true pathogen,
whereas E. dispar is used for the nonpathogenic species. Unfortunately, they look
identical morphologically. The only time E. histolytica could be identified morphologically would be from trophozoites containing ingested RBCs.
Nonpathogenic E. dispar would not contain ingested RBCs. The correct way to report
these organisms is E. histolytica/E. dispar (no trophozoites containing ingested RBCs)
or E. histolytica (trophozoites seen that contain ingested RBCs). Physicians may base
the treatment on patient symptoms.

Answer 388

A

B When eosinophils disintegrate, the granules reform into Charcot-Leyden crystals.

Answer 389

A

D Of the organisms listed, Diphyllobothrium latum cannot be transmitted sexually; this
infection is acquired from the ingestion of infected raw or poorly cooked freshwater
fish.

Answer 390

A

A East and West African trypanosomiasis (T. B. rhodesiense and T. B. gambiense) are
acquired when infective forms are introduced into the human body through the bite of
the tse-tse fly, not sand flea

Answer 391

A

C C. belli oocysts in various stages of maturity would be seen in the concentrate sediment
or possibly the direct, wet preparation; these oocysts would stain positive with
modified acid-fast stains

Answer 392

A

D T. vaginalis is site-specific. The organisms are found in the urogenital tract; thus, the
intestinal tract is not the normal site for these organisms.

Answer 393

A

B The best technique to recover and identify D. fragilis trophozoites and rare cysts would
be the trichrome-stained smear

Answer 394

A

C For a number of years, G. lamblia (G. intestinalis, G. duodenalis) has been implicated
in both waterborne and foodborne outbreaks from the ingestion of infective cysts
within contaminated water and food.

Answer 395

A

C E. gingivalis is known to be an inhabitant of the mouth and is characterized by
morphology that resembles E. histolytica/E. dispar. However, E. gingivalis tends to
ingest PMNs, whereas E. histolytica/E. dispar do not.

Answer 396

A

A The trophozoite of E. histolytica has evenly arranged chromatin on the nuclear
membrane; a central, compact karyosome in the nucleus; clear pseudopodia; and
ingested RBCs in the cytoplasm

Answer 397

A

D N. fowleri are free-living soil and water amoebae that cause primary amebic
meningoencephalitis (PAM). The number of cases reported is few; however, the
infection is very acute and almost always fatal.

Answer 398

A

A The rhabditiform larvae of S. stercoralis are characterized by the short buccal capsule
(mouth) and large genital primordium, whereas hookworm larvae have a long buccal
capsule and very small genital primordium.

Answer 399

A

A Toxocara spp. are the cause of visceral larva migrans and occur when humans
accidentally ingest the infective eggs of the dog or cat ascarid. The larvae migrate
through the deep tissues, including the eye. The test of choice is the serology.

Answer 400

A

D B. coli is a ciliate that can cause watery diarrhea in humans; however, it is not
commonly found within the United States. It is the largest of the intestinal protozoa
and can be found in proficiency testing specimens. Therefore, although it is not
common, laboratories must still be able to identify these organisms.

Answer 401

A

B S. haematobium blood flukes reside in the veins over the bladder. When the eggs are passed from the body, they are often found in urine; egg viability can also be
determined in unpreserved urine.

Answer 402

A

A P. westermani adult worms are found in the lung, and eggs may be coughed up in the
sputum. Consequently, both sputum and stool (if the sputum containing the eggs is
swallowed) are the recommended specimens for examination for the eggs.

Answer 403

A

A Both D. latum and P. westermani eggs are operculated and approximately the same
size. The morphology is similar, although D. latum has a knob at the abopercular end
and P. westermani has a thickened abopercular end and shoulders into which the
operculum fits

Answer 404

A

A Both T. solium (pork tapeworm) and T. spiralis can be acquired from the ingestion of
raw or poorly cooked infected pork.

Answer 405

A

B D. latum is the only operculated cestode egg that is found in humans; the infection is
acquired from the ingestion of infected raw or poorly cooked freshwater fish.

Answer 406

A

A Although hydatid cysts are found in sheep or in humans (accidental intermediate
host), the adult tapeworms of E. granulosus are found in the intestine of the dog.

Answer 407

A

C If humans ingest T. solium cysticerci in uncooked or rare pork, the adult tapeworm
will mature within the intestine (human will serve as definitive host); if eggs from the
adult tapeworm are ingested, then the cysticerci will develop in human tissues
(accidental intermediate host), causing cysticercosis.

Answer 408

A

D The ingestion of raw freshwater fish containing the encysted larvae of D. latum will
result in the development of an adult tapeworm within the human intestine.

Answer 409

A

B In infections with H. nana, humans serve as both intermediate and definitive hosts.
When ingested, the oncosphere penetrates the intestinal mucosa, develops into the
mature cysticercoid (human is intermediate host), and returns to the gut, where the
adult tapeworm matures (human is definitive host). Thus, the life cycle can go from
egg to adult tapeworm within the human host

Answer 410

A

A Babesia is an intracellular parasite that closely resembles the ring forms (early
trophozoites) of P. falciparum. Often in babesiosis there are more rings per cell and the
ring form is the only stage seen

Answer 411

A

C A. lumbricoides eggs require a period of development in the soil before they are
infective for humans. The other organisms listed can be transmitted within the
laboratory or in the hospital setting.

Answer 412

A

D Infection with T. gondii is acquired through the ingestion of infected rare or raw
meats, infective oocysts from cat feces, or as a congenital transmission. The organism has a cosmopolitan distribution and although serological testing is generally the test of
choice, the results may be very difficult to interpret in certain situations (e.g.,
congenital infection and immunocompromised patients).

Answer 413

A

A Oocysts of Cryptosporidium spp. can be found and identified using microscopic
examination of fecal smears stained with modified acid-fast stains. They appear as
purple-red-pink round objects, measuring approximately 4 to 6 μm. Often the four
sporozoites and residual body can be seen within the oocyst wall

Answer 414

A

C O. volvulus microfilariae are normally found in the fluid right under the outer layer of
skin. Therefore, the skin snip is the proper specimen to examine.

Answer 415

A

B The pathogenic sequelae of malarial infections with P. falciparum are the most severe
of the four species listed. They can include massive hemolysis, blackwater fever, and
multiple organ involvement, including the central nervous system (cerebral malaria).

Answer 416

A

B G. lamblia (G. intestinalis, G. duodenalis), D. fragilis, and Cryptosporidium have
been implicated in nursery school outbreaks. Among the many protozoa and coccidia
found in the human, these three organisms have become the most likely parasites in
this type of setting

Answer 417

A

D Infections with E. vermicularis (the pinworm) may cause anal itching, sleeplessness,
and possibly some vaginal irritation or discharge; however, dysentery (bloody
diarrhea) has not been associated with this infection.

Answer 418

A

B The ova and parasite examination contains three components: the direct wet film
(demonstrates protozoan trophozoite motility), the formalin-ethyl acetate concentration
(demonstrates protozoan cysts, coccidian oocysts, and helminth eggs), and the
trichrome or iron hematoxylin–stained smear (confirms protozoan cysts and
trophozoites

Answer 419

A

C The cyst of I. bütschlii is characterized by a large glycogen vacuole that is seen on the
wet smear (stains brown with iodine) and on the permanent stained smear (vacuole will
appear clear). Occasionally the vacuole will be so large that the organism will collapse
on itsel

Answer 420

A

B The determination of egg viability is important in schistosomiasis; therefore, the
miracidial hatching test is helpful in demonstrating the egg viability of Schistosoma
species. Once the eggs are hatched, the living miracidium larvae will be visible in the
water

Answer 421

A

A Both G. lamblia (G. intestinalis, G. duodenalis) and Cryptosporidium have been
implicated in waterborne outbreaks or diarrheal disease. These infections would result
from the ingestion of Giardia cysts and/or Cryptosporidium oocysts

Answer 422

A

B Fecal immunoassays have become more commonly used for the diagnosis of
infections with Giardia and Cryptosporidium. These tests are available in EIA, FA, or rapid cartridge formats

Answer 423

A

C With the possible exception of direct inoculation infection in the eye, the
microsporidia are thought to initially infect the gastrointestinal (GI) tract through
ingestion of the infective spores; infections in other body sites are thought to
disseminate from the GI tract.

Answer 424

A

C The majority of eye infections with Acanthamoeba spp. have resulted from the use of
contaminated eye care solutions, primarily the use of homemade saline. It is
recommended that all solutions be discarded at the expiration date. Continued use may
increase the risk of environmental contamination of the fluids

Answer 425

A

C Currently, the most sensitive method of those listed for the recovery of
Acanthamoeba spp. from clinical specimens is the non-nutrient agar culture plate
seeded with E. coli. The amoebae feed on the bacteria; both trophozoites and cysts can
be recovered from the agar surface

Answer 426

A

A Although the microsporidia have been known as pathogens in many groups of
animals, their involvement in humans has primarily been in immunocompromised
patients, especially those with AIDS. Microsporidia can be found in different tissues,
and currently, there are a number of genera implicated in human disease.

Answer 427

A

B The decolorizer in modified acid-fast stains (Kinyoun cold method, modified hot
method) is usually 1% to 3% sulfuric acid rather than the much stronger acid alcohol
used in the routine AFB stains

Answer 428

A

D T. trichiura (whipworm) may cause diarrhea and occasionally dysentery in very
heavy infections; however, the worms are confined to the intestine, and myalgias are
not seen in this helminth infection.

Answer 429

A

C H. nana has a thin eggshell containing a six-hooked embryo (oncosphere) and polar
filaments that lie between the eggshell and the embryo. The striated eggshell is
generally associated with the eggs of Taenia spp.

Answer 430

A

B The Knott concentration is designed to allow the recovery of microfilariae from a
blood specimen. Dilute formalin (2%) is used; blood is introduced into the formalin,
the red cells lyse, and the sediment can be examined as a wet preparation or permanent
stained smear (Giemsa or hematoxylin-based stain) for the presence of microfilariae.

Answer 431

A

D The egg of S. mansoni is characterized by a large lateral spine; S. haematobium has
the characteristic terminal spine.

Answer 432

A

C The request for blood films for malaria should always be considered a STAT request.
Any laboratory providing these services should have coverage 24 hours a day, 7 days a
week. In cases of P. falciparum malaria, any delay in diagnosing the infection could be
fatal for the patient.

Answer 433

A

B A latent infection with S. stercoralis acquired years before may cause severe
symptoms in the immunosuppressed patient (“autoinfective” capability of life cycle
and migratory route through the body leading to the hyperinfection syndrome and
disseminated disease).

Answer 434

A

D The collection of a series of consecutive Scotch/cellophane tape preparations is
recommended for the diagnosis of infection with the pinworm E. vermicularis.

Answer 435

A

B Eosinophilic meningoencephalitis is a form of larva migrans and is caused by A.
cantonensis, the rat lungworm. This infection, common in the Pacific region, is associated with CSF symptoms and sometimes eye involvement

Answer 436

A

D Duplicate cultures should be set up, and specific American Type Culture Collection
(ATCC) strains should be cultured along with the patient specimens to confirm that the
culture system is operating properly. This approach is somewhat different from that
used in diagnostic bacteriology and mycology.

Answer 437

A

C As E. coli cysts mature, the cyst wall becomes more impenetrable to fixatives.
Consequently, the cysts may be visible in the concentrate sediment but appear very
distorted or pale on the permanent stained smear.

Answer 438

A

A The cause of hydatid disease is E. granulosus, and the human is classified as the
accidental intermediate host. Infection occurs when humans accidentally ingest the
eggs of E. granulosus and the hydatid cysts develop in the liver, lung, and other organs
of the human instead of sheep (normal cycle).

Answer 439

A

D Bear meat is another excellent source of T. spiralis. In this case, the patient had
evidently consumed poorly cooked infected bear meat, thus ingesting the encysted
larvae of T. spiralis.

Answer 440

A

B The accidental ingestion of T. solium eggs can result in the disease called
cysticercosis. The cysticerci will develop in a number of different tissues, including the
brain, and humans are accidental intermediate hosts.

Answer 441

A

B The fact that the patient has received a transplant, is on immunosuppressive drugs,
and has increasing diarrhea, suggests infection with microsporidia; the most
appropriate method would be the modified trichrome stain

Answer 442

A

D On the basis of the history, the most relevant procedure to perform is the preparation
and examination of thick blood films for the recovery and identification of
microfilariae. The symptoms suggest early filariasis

Answer 443

A

C Cryptosporidium oocysts (unlike those of C. belli) are immediately infective when
passed in stool, and nosocomial infections have been well documented with this
coccidian

Answer 444

A

D T. cruzi, the cause of Chagas disease, exists in two forms within humans, the
trypomastigote in the blood and the amastigote in the striated muscle (usually cardiac
muscle and intestinal tract muscle

Answer 445

A

C Travel history (areas of drug resistance), the date of return to the United States
(primary versus relapse case), and history of antimalarial medication and illness
(severe illness, few organisms on smear) are very important questions to ask. Without
this information, a malaria diagnosis can be missed or delayed with severe patient
consequence

Answer 446

A

A Cryptosporidium can be transmitted through contaminated municipal water sources.
Such outbreaks have been well documented

Answer 447

A

B The coccidian C. cayetanensis has been linked to mini-outbreaks of diarrheal disease.
Epidemiological evidence strongly implicates various berries, basil, mesclun, and snow
peas as likely causes. These outbreaks are very sporadic and tend to occur primarily in
March through May, primarily from imported foods.

Answer 448

A

C The adult O. volvulus reside in subcutaneous nodules, and the microfilariae are found in the fluids right under the outer layers of skin; thus the appropriate diagnostic test is
the microscopic examination of skin snips for the presence of microfilariae

Answer 449

A

A The pinworm, E. vermicularis, is the most common parasitic infection throughout the
world, and the eggs are infective within just a few hours. It has been said, “You either
had the infection as a child, have it now, or will have it again when you have children.”

Answer 450

A

B The eggs of T. trichiura (the whipworm) have been described as being barrel shaped,
with a thick shell and two polar plugs, one at each end of the egg.

Answer 451

A

C The eggs of T. canis are infectious for humans and cause visceral and ocular larva
migrans. These ascarid eggs of the dog can infect humans; the eggs hatch and the
larvae wander through the deep tissues, occasionally the eye. In this case, the human
becomes the accidental intermediate host.

Answer 452

A

A Both P. vivax and P. ovale infect young RBCs, have true stippling (Schüffner dots),
contain malarial pigment, and have a true relapse stage in the life cycle

Answer 453

A

C Both Cryptosporidium and Strongyloides have the capability for internal autoinfection
in their life cycles. This means that the cycle and infection can continue even after the
patient has left the endemic area. In the case of Cryptosporidium, the cycle continues
in patients who are immunocompromised and incapable of self cure.

Answer 454

A

D Infectious routes for microsporidial infections have been confirmed as ingestion and
inhalation of the spores; direct transfer of infectious spores from environmental
surfaces to the eyes has also been reported

Answer 455

A

D Routine O&P examinations usually do not allow the detection of Cryptosporidium
oocysts and microsporidial spores; special stains are required. Modified acid-fast stains
for Cryptosporidium and modified trichrome stains for the microsporidia are
recommended

Answer 456

A

C Evidence of the cross/diagonal line polar tubules provides confirmation for
microsporidial spores in the clinical specimen. Not all spores will contain visible polar
tubules; the stain of choice is the modified trichrome stain (Ryan/blue or
Weber/green

Answer 457

A

D Early ring forms of the fifth human malaria, P. knowlesi, resemble most closely ring
forms of P. falciparum. However, in the early ring form stage, all human malaria
species tend to look very similar

Answer 458

A

D On passage in stool from the gastrointestinal tract, the oocysts of Cryptosporidium are
immediately infective; nosocomial infections have been documented with this
organism. The oocysts of other coccidia have to undergo further development before
they are infective

Answer 459

A

C Older developing stages (developing trophozoites, schizonts) of the fifth human
malaria, P. knowlesi, tend to closely resemble those of P. malariae (band forms, “daisy
head” mature schizont

Answer 460

A

D Autofluorescence is often used in preliminary testing for the presence of C.
cayetanensis oocysts (8–10 μm). If positive, then the specimen can be stained using
modified acid-fast stains for confirmation

Answer 461

A

A Characteristics of the fifth human malaria, P. knowlesi, include a rapid erythrocytic
cycle of 24 hours, possible infection of all ages of RBCs (as with P. falciparum), and
the potential for serious disease (as with P. falciparum).

Answer 462

A

A The use of a modified trichrome stain (Ryan/blue, Weber/green; both with a very high
dye content) and light microscopy (oil immersion using the 100× objective) can be
used to confirm a microsporidial infection

Answer 463

A

B Although the pathogenicity of Blastocystis spp. continues to be debated (some
patients are symptomatic, and some are asymptomatic), it is now known that the
organisms we see microscopically comprise a number of subtypes/strains/subspecies.
Approximately half these subtypes are pathogenic for humans, and half are
nonpathogenic; however, morphologically they all look the same. Correct reporting of
this infection should be Blastocystis spp.; numbers should also be reported (rare, few,
moderate, many, packed

Answer 464

A

D All of the changes mentioned are seen in a tube of EDTA blood that has been held too
long prior to blood film preparation. Many morphological changes of the parasites, as
well as lost organisms, occur under these conditions. These blood specimens should
always be handled STAT (ordering, collection, processing, examination, and reporting).

Answer 465

A

D With thick blood films, the RBCs must be ruptured/laked either prior to or during
staining. Without laking the RBCs, the stained smears would be too thick or too dark
to be able to visualize any morphology. In contrast, the thin blood film is fixed prior
to/during staining to preserve the RBC morphology; the parasite morphology within
the infected RBC can be reviewed for possible diagnosis to the species level. The thin
film has a much smaller amount of blood than the thick blood film.

Answer 466

A

A CNA inhibits most strains of S.saprophyticus. Therefore, blood agar should be used
when culturing catheterized urine samples from young female patients. Most S.
saprophyticus isolates are obtained from female patients ages 20 to 30 years.

Answer 467

A

C The high concentration of NaCl (7.5%) in mannitol salt agar allows for the recovery of
S. aureus from heavily contaminated specimens while inhibiting other organisms.
Furthermore, S. aureus ferments mannitol, thus allowing for easy detection of yellowhaloed
colonies of S. aureus on red mannitol salt agar

Answer 468

A

B Generally, tube 2 or 3 is submitted to the microbiology laboratory for culture and Gram
staining. To ensure recovery of any pathogens and correct diagnosis without
contamination by other bacteria, immediate centrifugation and inoculation to the
appropriate media, as well as Gram staining, should be performed prior to delivery of
the specimen to the other departments for testing.

Answer 469

A

D In general, a 24-hour urine is unsuitable for culture; a first morning specimen is best
for the recovery of mycobacteria in the urine.

Answer 470

A

B Specimens submitted for culture and recovery of any bacteria should be submitted
without fixatives

Answer 471

A

D EIEC produces dysentery similar to that by Shigella, with invasion and destruction of
the intestinal mucosal epithelium. Leukocytes are seen on the Gram-stained smear.
Adults who travel to foreign countries, especially Mexico, are at greatest risk.

Answer 472

A

A K. oxytoca is similar to K. pneumoniae except that the indole test is positive for K.
oxytoca.

Answer 473

A

B Y. enterocolitica has been associated with fatal bacteremia and septic shock from
contaminated blood transfusion products. The motility at room temperature is a clue to
this identification

Answer 474

A

B E. coli O157:H7 is usually the most common isolate from bloody stools of the EHEC
group, which results from undercooked beef. These strains are waterborne and
foodborne, and the infections from E. coli O157:H7 are greatest during the summer
months in temperate climates.

Answer 475

A

A Y. enterocolitica is responsible for diseases in younger persons. Blood and leukocytes
can be present in stools. Patients (usually teens) exhibiting appendicitislike symptoms
with lactose-negative colonies growing on MacConkey agar (small colonies at 24
hours, but larger colonies at 48 hours if incubated at room temperature) should be
tested for the growth of Y. enterocolitica.

Answer 476

A

A S. maltophilia is the third most frequently isolated nonfermentative gram-negative rod
in the clinical laboratory. Patients with CF are at greater risk for infections because of
previous antimicrobial treatment and recurrent pneumonia and because

Answer 477

A

D E. corrodens is part of the normal flora of the human mouth and typically “pits” the
agar. This organism is capnophilic (needing increased CO2).

Answer 478

A

B C. canimorsus is associated with septicemia or meningitis following dog bites. All
Capnocytophaga strains are capnophilic, facultative anaerobic, gram-negative, slender
or filamentous rods with tapered ends

Answer 479

A

B Both E. coli O157:H7 and Shigella spp. are invasive and cause bloody stools

Answer 480

A

A S. aureus is an opportunistic human pathogen. A wound or ulcer infected with S.
aureus that is left untreated is especially detrimental to a patient with diabetes

Answer 481

A

C MRSA isolates are usually tested for susceptibility or resistance to vancomycin, a
glycopeptide

Answer 482

A

B Group B streptococci (S. agalactiae) are important pathogens and can cause serious
neonatal infections. Women who are found to be heavily colonized vaginally with S.
agalactiae pose a threat to the newborn, especially within the first few days after
delivery. The infection acquired by the infant is associated with pneumonia

Answer 483

A

C Enterococcus (Streptococcus) faecalis is the cause of up to 20% of the bacterial
endocarditis cases and is the most commonly encountered species in this condition

Answer 484

A

A N. gonorrhoeae can be presumptively identified from a male patient (only) from
Gram staining and growth on selective agar. In female patients, the normal flora from a
urethral swab may appear to be N. gonorrhoeae (gram-negative diplococci) but may be
part of the normal flora, such as Veillonella spp. (anaerobic gram-negative cocci
resembling N. gonorrhoeae).

Answer 485

A

C G. vaginalis, a gram-negative or gram-variable pleomorphic coccobacillus, causes
bacterial vaginosis, but is also present as part of the normal vaginal flora of women of
reproductive age with a normal vaginal examination. “Clue cells” are vaginal epithelial
cells with gram-negative or gram-variable coccobacilli attached to them.

Answer 486

A

A L. monocytogenes is catalase positive and displays a “tumbling” motility at room
temperature. Streptococcus spp. are catalase negative and nonmotile.

Answer 487

A

A M. tuberculosis is niacin positive and nonphotochromogenic. This organism takes up
to 3 weeks to grow on selective agar.

Answer 488

A

A A swarmer on blood agar would most likely be a Proteus spp. A lactose nonfermenter
and swarmer that is often isolated from UTIs is P. mirabilis.

Answer 489

A

B P. aeruginosa has a distinctive grape odor. The best tests to differentiate it from
Acinetobacter spp. are growth at 42°C, oxidase and motility which give the following
results

Answer 490

A

C H. haemolyticus requires both X and V factors for growth and lyses horse erythrocytes.

Answer 491

A

A C. perfringens is an anaerobic gram-positive rod that is often isolated from the tissue
of patients with gas gangrene (myonecrosis). Spore production is not usually seen with
this organism, which may also stain gram-negative.

Answer 492

A

B B. fragilis is the most frequently isolated gram-negative anaerobic bacillus. It is
resistant to many antibiotics. A good screening agar is a 20% bile plate that does not
support the growth of Prevotella spp. or Porphyromonas spp.

Answer 493

A

B Breakpoint susceptibility testing is done by selecting only two appropriate drug
concentrations for testing. If the results show growth at both concentrations, then
resistance is indicated; growth only at the lower concentration signifies an intermediate
result; no growth at either concentration is interpreted as susceptible.

Answer 494

A

B Elizabethkingia (formerly Chryseobacterium) meningosepticum is a well-known
cause of neonatal meningitis. It grows well on chocolate agar, producing yellowpigmented
colonies

Answer 495

A

A L. pneumophila is the cause of pneumonia and can occur as part of an epidemic
sporadically or as a nosocomial infection or may be community acquired. The
appearance of mottled, cut-glass colonies on buffered CYE agar under low power and
the use of a DIF technique on sputum samples determine the presence of L.
pneumophila. The most common environmental sites for recovery are shower heads,
faucets, water tanks, and air-conditioning systems.

Answer 496

A

A Leuconostoc spp. are vancomycin-resistant opportunistic pathogens, and infection
with them occurs after invasive procedures. The organisms are often recovered from
positive neonatal blood culture specimens following colonization during delivery.

Answer 497

A

B Gemella spp. are often recovered from patients with endocarditis and meningitis. In
the Gram-stained smear, they resemble staphylococci morphologically but are catalase
negative.

Answer 498

A

A Leuconostoc spp. and Pediococcus spp. are vancomycin-resistant, catalase-negative,
gram-positive aerobic organisms recovered from immunosuppressed patients.

Answer 499

A

D S. aureus is a usual cause of skin infections and a common cause of cellulitis,
impetigo, postsurgical wounds, and scalded skin syndrome in infants.

Answer 500

A

B S. aureus is the cause of “scalded skin” syndrome in newborn infants. The production
of a potent exotoxin (exfoliatin) causes the epidermis to slough off, leaving the
newborn’s skin with a red, raw texture or a burned, scalded look.

Answer 501

A

A E. faecalis gives a positive reaction to the PYR test and is often implicated in UTIs. It
is part of the normal flora of the female genitourinary tract and the human
gastrointestinal tract. On 5% sheep blood agar, E. faecalis colonies may appear as
nonhemolytic, α-hemolytic, or β-hemolytic colonies, depending on the strain

Answer 502

A

C S. pneumoniae colonies appear as α-hemolytic donut-shaped colonies on 5% sheep
blood agar. The mucoid colonies may appear “wet” or “watery” because of the capsule
surrounding the organism. The Gram stain smear reveals lancet-shaped gram-positive
cocci in pairs surrounded by a clear area (the capsule). To differentiate the viridans
streptococci from S. pneumoniae, the most appropriate test is the optochin disk test. S.
pneumoniae on blood agar are susceptible to optochin, but viridans streptococci are
resistant.

Answer 503

A

B H. pylori is not easily cultured for growth and identification. Tissue samples should
be transported in appropriate media and tested immediately. Other means of successful
identification are rapid urease test on biopsy material; urea breath test; and serological
tests for the detection of antibodies to H. pylori by ELISA and IFA procedures

Answer 504

A

A L. monocytogenes colonies recovered from blood and CSF display a narrow zone of
β-hemolysis on 5% sheep blood agar, which often mimics group B β-hemolytic
streptococci. A catalase test and Gram staining will differentiate the two organisms. L.
monocytogenes are catalase positive, motile (tumbling motility at room temperature),
and rod shaped instead of being coccus shaped.

Answer 505

A

C C. diphtheriae, unlike other Corynebacterium spp., are not part of the normal flora of
the human nasopharynx. Exposure through direct contact (respiratory or cutaneous
lesions) is the most likely mode of transmission. Underdeveloped countries are the
prime areas where exposure to C. diphtheriae occurs. The diagnosis is made more
rapidly when the examining physician alerts the laboratory that diphtheria is suspected
so that Tinsdale agar or other media containing tellurite salts can be used for culture.
Corynebacterium will grow on blood and chocolate agars, but Tinsdale agar is the
preferred culture medium because the potassium tellurite in the agar causes C.
diphtheriae to produce brown colonies surrounded by a brown halo. The halo effect is
seen with C. diphtheriae, C. ulcerans, and C. pseudotuberculosis, but not with other
Corynebacterium or with other pigmented colonies growing on Tinsdale agar, such as
Streptococcus or Staphylococcus spp

Answer 506

A

B G. vaginalis is part of the normal flora (anorectal) of adults and children. Clue cells
(vaginal epithelial cells with gram-variable coccobacilli on the cell margins) are seen
in vaginal washings and the organism grows slowly on chocolate agar, 5% sheep blood
agar, and V-agar.

Answer 507

A

A N. asteroides is a gram-positive, beaded, long bacillus and is partially acid fast. It is
an intracellular pathogen that grows in human cells. Immunocompromised patients are
susceptible to infections, especially pulmonary infections, which then disseminate to
other organs, often proving fatal

Answer 508

A

A Early detection in pregnant women is very important when dealing with L.
monocytogenes. If it is not detected and treated, infection of the fetus, resulting in
stillbirth, abortion, or premature birth may result. Detection can also be made
postpartum by culturing the CSF, blood, amniotic fluid, and respiratory secretions of
the neonate

Answer 509

A

D The overgrowth of C. difficile in the bowel is the cause of antimicrobial-associated
colitis. Culturing for C. difficile is less specific for infection than toxin testing, but is
the more sensitive method, along with NAT, to detect possible disease related to C.

Answer 510

A

B P. acnes is a diphtheroid (pleomorphic rod) that may appear to branch on the Gramstained
smear. It is one of the most common organisms isolated from blood cultures
and is often a contaminant. Abundant propionic acid is produced by GLC.

Answer 511

A

A C. septicum is often recovered from patients with malignancies or other diseases of
the colon, especially the cecum. The following chart defines the swarming Clostridium
spp.

Answer 512

A

B The B. fragilis group is a dominant part of the indigenous flora of the large bowel and
is recovered most commonly from postoperative abdominal fluids. The B. fragilis
group is more resistant to antibiotics and is not pigmented. Prevotella and
Porphyromonas spp. are pigmented.

Answer 513

A

A A slender gram-negative rod with pointed ends that does not grow on 20% bile agar
rules out B. fragilis group and indicates F. nucleatum.

Answer 514

A

B C. albicans is the common cause of oral thrush involving the mucocutaneous
membranes of the mouth. C. albicans is part of the normal flora of skin, mucous
membranes, and the gastrointestinal tract.

Answer 515

A

D A. fumigatus is the cause of aspergillosis and involves the organism colonizing the
mucous plugs in the lung. This is called allergic aspergillosis and is characterized by a
high titer of IgE antibody to Aspergillus. Invasive aspergillosis seen in patients with
neutropenia exhibits sinusitis and is disseminated throughout the body.

Answer 516

A

A T. mentagrophytes, a common cause of athletes’ foot, is sometimes confused with T.
rubrum, the most common dermatophyte to infect humans. The differential tests are
shown in the following chart

Answer 517

A

C C. neoformans produces brown colonies on birdseed agar, is urease positive, and
produces only blastospores. Immunosuppressed patients are susceptible to this
organism.

Answer 518

A

A CMV infection in young adults causes a self-limiting mononucleosis syndrome. CMV
infections are common and usually self-limiting, except in neonates and
immunosuppressed patients, in whom they may cause a life-threatening situation.

Answer 519

A

B With the permission of the patient (state law may require him or her to sign a consent
form) and after counseling of the student nurse, the appropriate course of action is to
test the patient for HIV by using a screening test (EIA or ELISA). The student should
also be tested for baseline values. If the HIV test result is positive for the patient, the
student is administered the appropriate antiviral drug(s) immediately or within 2 hours
of the incident. Confirmatory testing should be done after an initial positive HIV test
result.

Answer 520

A

B Herpes genitalis is an infection caused by HSV-2. Symptomatic primary herpes by HSV-2 is responsible for about 85% of herpes infections. HSV-1 (causing the other
15%) does not involve recurring infections of herpes and causes fever blisters. HSV-2
causes 99% of recurrent genital herpes.

Answer 521

A

D If the patient has malaria and has been taking prophylaxis (often sporadically), the
number of parasites on the blood smear will be reduced and examination of routine
thick and thin blood films should be more exhaustive. Furthermore, specific
geographic travel history may help to determine whether chloroquine-resistant P.
falciparum may be a factor. Also, if the patient is immunologically naïve (no prior
exposure to malaria), these patients usually come into an emergency room with a very
low parasitemia and no periodicity; thus malaria may not even be suspected

Answer 522

A

C One of the coccidian parasites, C. cayetanensis, has been implicated in cases of
human diarrhea. The recommended stains are modified acid-fast stains, and the
organisms are quite variable in their staining characteristics. The color can range from
clear (resembling wrinkled cellophane) to red-purple. The oocysts are immature when
passed (no internal morphology) and they measure about 8 to 10 μm

Answer 523

A

A It is now well established that E. histolytica is being used to designate the true
pathogenic species, whereas E. dispar is now being used to designate nonpathogenic
species. Unless trophozoites containing ingested RBCs (E. histolytica) are seen, the
two organisms cannot be differentiated on the basis of morphology. On the basis of
this information, there are now two separate species, only one of which (E. histolytica)
is pathogenic. Because this patient is asymptomatic, the organisms seen in the fecal
smears are probably E. dispar (nonpathogen); the laboratory report should have said
“Entamoeba histolytica/Entamoeba dispar”—unable to differentiate on the basis of
morphology unless trophozoites are seen to contain ingested RBCs (E. histolytica—
true pathogen). Immunoassays and real time PCR can differentiate the true pathogen,
E. histolytica, from nonpathogenic species of Entamoeba, and are more sensitive and
specific than microscopic examination alone.

Answer 524

A

B It is well known that G. lamblia (G. intestinalis, G. duodenalis) trophozoites adhere to
the intestinal mucosal surface by means of the sucking disk. Although a patient may
have giardiasis and be symptomatic, confirmation of the infection from stool
examinations may require more than the routine three stools or may require the
examination of duodenal contents. These organisms are shed very sporadically. Two
fecal immunoassays may be required to diagnose giardiasis; this also relates to
sporadic shedding. The need for two fecal immunoassays does not apply to
cryptosporidiosis; a single immunoassay is sufficien

Answer 525

A

C Although infection with S. stercoralis may have been acquired in Central America
many years before, the patient may have remained asymptomatic while the infection
was maintained at a low level in the body via the autoinfective portion of the life cycle.
As the patient became more immunosuppressed (steroids), the life cycle began to
reactivate with penetration of the larvae through the intestinal wall (abdominal pain)
and larval migration through the lungs (pneumonia), and the patient may have
presented with evidence of sepsis (often with gram-negative bacteria carried with the
larvae as they penetrate the intestinal wall). In patients who become
immunosuppressed, the life cycle of Strongyloides may be reactivated, and this results
in serious illness (e.g., hyperinfection syndrome, disseminated disease); this can occur
many years after the initial infection and after the patient has left the endemic area.

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