Microbiology and Parasitology Problem Solving

Question 1

An emergency department physician ordered a culture and sensitivity test on a catheterized urine specimen obtained from a 24-year-old female
patient. A colony count was done and gave the following results after 24 hours:
Blood agar plate = >100,000 col/mL of gram-positive cocci resembling staphylococci
MacConkey agar = No growth
CNA plate = Inhibited growth
Hemolysis = Neg Catalase = Positive
Novobiocin = Resistant
This isolate is:
A. Staphylococcus saprophyticus
B. Micrococcus luteus
C. Staphylococcus aureus
D. Streptococcus pyogenes

Answer 1

A. Staphylococcus saprophyticus

Note: 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 20–30 years old.

Question 2

An outbreak of Staphylococcus aureus in the nursery department prompted the Infection Control Committee to proceed with an environmental screening procedure. The best screening media to use for this purpose would be:
A. CNA agar
B. THIO broth
C. Mannitol salt agar
D. PEA agar

Answer 2

C. Mannitol salt agar

Note: 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. Also, S. aureus ferments mannitol, thus allowing for easy detection of yellow-haloed colonies of S. aureus on red mannitol salt agar.

Question 3

A listless 12-month-old boy with a fever of 103°F was taken to the emergency department. He had been diagnosed with an ear infection 3 days earlier. A spinal tap was performed, but only one tube of CSF was obtained from the lumbar puncture. The single tube of CSF should be submitted first to which department?
A. Chemistry
B. Microbiology
C. Hematology
D. Cytology/Histology

Answer 3

B. Microbiology

Note: Generally, tube 2 or 3 is submitted to the microbiology laboratory for culture and Gram stain smear. To ensure recovery of any pathogens and correct diagnosis without other bacterial contamination, immediate centrifugation and inoculation to the appropriate media as well as a Gram stain smear should be performed prior to delivery of the specimen to the chemistry department for testing.

Question 4

A 65-year-old female outpatient was requested by her physician to submit a 24-hour urine specimen for protein and creatinine tests. He also requested testing for mycobacteria in the urine. Should the microbiology laboratory accept this 24-hour specimen for culture?
A. Yes, if the specimen is kept on ice
B. Yes, if the specimen is for aerobic culture only
C. No, the specimen must be kept at room temperature
D. No, the specimen is unsuitable for the recovery of mycobacteria

Answer 4

D. No, the specimen is unsuitable for the recovery of mycobacteria

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

Question 5

A lymph node biopsy obtained from a 30-year-old male patient was submitted to the microbiology
laboratory for a culture and AFB smear for mycobacteria. The specimen was fixed in formalin. This specimen should be:
A. Accepted for AFB smear and cultured
B. Rejected
C. Held at room temperature for 24 hours and then cultured
D. Cultured for anaerobes only

Answer 5

B. Rejected

Note: Specimens submitted for culture and recovery of any bacteria should be submitted without fixatives.

Question 6

A 49-year-old man who traveled to Mexico City returned with a bad case of dysentery. His symptoms were fever; abdominal cramping; and
bloody, mucoidal, frequent stools. In addition to this, many WBCs were seen on the Gram stain smear. Stool culture gave the following results:
Gram stain: Gram-negative rods Lactose = +
Indole = +
Lysine decarboxylase = Neg
Urease = Neg Motility = Neg
What is the most likely organism?
A. Salmonella spp.
B. Proteus mirabilis
C. Escherichia coli
D. Enteroinvasive E. coli (EIEC)

Answer 6

D. Enteroinvasive E. coli (EIEC)

Note: EIEC, or enteroinvasive E. coli, produces dysentery similar to that of Shigella, with invasion and destruction of the intestinal mucosal epithelium.
Leukocytes are seen on the Gram stain smear. Adults who are travelers to foreign countries, especially Mexico, are at greatest risk.

Question 7

An 80-year-old male patient was admitted to the hospital with a fever of 102°F. A sputum culture revealed many gram-negative rods on MacConkey agar and blood agar. The patient was diagnosed with pneumonia. The following biochemical results were obtained from the culture:
H2S = Neg Lactose = + Urease = +
Citrate = + Indole = + VP = +
Motility = Neg Resistance to ampicillin
and carbenicillin
What is the most likely identification?
A. Klebsiella oxytoca
B. Proteus mirabilis
C. Escherichia coli
D. Klebsiella pneumoniae

Answer 7

A. Klebsiella oxytoca

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

Question 8

An immunocompromised 58-year-old female chemotherapy patient received 2 units of packed RBCs. The patient died 3 days later, and the report from the autopsy revealed that her death was due to septic shock. The blood bags were cultured, and
the following results were noted:
GROWTH OF AEROBIC GRAM-NEGATIVE RODS ON BOTH MACCONKEY AND BLOOD AGARS
Lactose = Neg Sucrose = +
Citrate = Neg Indole = Neg VP = Neg H2S = Neg Urease = +
Motility 22°C = + Motility 37°C = Neg
What is the most likely identification?
A. Escherichia coli
B. Yersinia enterocolitica
C. Enterobacter cloacae
D. Citrobacter freundii

Answer 8

B. Yersinia enterocolitica

Note: 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.

Question 9

A pediatric patient with severe bloody diarrhea who had been camping with his parents was admitted to the hospital with complications of hemolytic uremic syndrome (HUS). Several stool specimens were cultured with the following results noted:
Gram stain smear = Many gram-negative rods with no WBCs seen
Blood agar = Normal flora
MacConkey agar = Normal flora
MacConkey agar with sorbitol = Many clear colonies (sorbitol negative)
Hektoen agar = Normal
Campy agar = No growth flora
What is the most likely identification?
A. Yersinia spp.
B. E. coli O157:H7
C. Salmonella spp.
D. Shigella spp.

Answer 9

B. E. coli O157:H7

Note: E. coli O157:H7 is usually the most common isolate from bloody stools of the enterohemorrhagic
E. coli (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.

Question 10

A 14-year-old emergency department patient had been to the doctor’s office 2 days previously with abdominal pain, diarrhea, and a low-grade fever.
He was diagnosed with pseudoappendicular syndrome. Cultures from the stool containing
blood and WBCs showed the following results:
AEROBIC GRAM-NEGATIVE RODS ON MACCONKEY AGAR (CLEAR COLONIES)
Campy agar = No growth
Lactose = Neg Sucrose = +
Citrate = Neg
Indole = Neg VP = Neg H2S = Neg
Motility 37°C = Neg Motility 22°C = + Hektoen agar = NF
What is the most likely identification?
A. Yersinia enterocolitica
B. Salmonella spp.
C. Shigella spp.
D. Escherichia coli

Answer 10

A. Yersinia enterocolitica

Note: Y. enterocolitica is responsible for diseases in younger persons. Blood and leukocytes can be present in stools. Patients (usually teens) exhibiting appendicitis-like 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.

Question 11

A sputum culture from a 13-year-old cystic fibrosis patient grew a predominance of short, gram-negative rods that tested oxidase negative.
On MacConkey, chocolate, and blood agar plates, the organism appeared to have a lavender-green pigment. Further testing showed:
Motility = + DNase = +
Glucose = + (oxidative)
Maltose = + (oxidative)
Lysine decarboxylase = +
Esculin hydrolysis = +
What is the most likely identification?
A. Stenotrophomonas maltophilia
B. Acinetobacter baumannii
C. Pseudomonas aeruginosa
D. Burkholderia (P.) cepacia

Answer 11

A. Stenotrophomonas maltophilia

Note: S. maltophilia is the third most frequently isolated nonfermentative gram-negative rod in the clinical
laboratory. Cystic fibrosis patients are at greater risk for infections because of previous antimicrobial treatment and recurrent pneumonia and because some strains may be colonizers.

Question 12

A patient with a human bite wound on the right forearm arrived at the clinic for treatment. The wound was inflicted 36 hours earlier, and a
culture was taken by the physician on duty. After 48 hours, the culture results were:
Gram-stain smear = Gram-negative straight, slender rods
Chocolate agar plate = “Pitting” of the agar by small, yellow, opaque colonies
Oxidase = + Motility = Neg
Catalase = Neg Glucose = +
Growth in increased CO2 = +
Growth at 42°C = Neg
What is the most likely identification of this facultative anaerobe?
A. Pseudomonas aeruginosa
B. Acinetobacter baumannii
C. Kingella kingae
D. Eikenella corrodens

Answer 12

D. Eikenella corrodens

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

Question 13

A dog bite wound to the thumb of a 20-year-old male patient became infected. The culture grew a gram-negative, slender rod, which was a facultative anaerobe. The following results were noted:
Oxidase = + Motility = Neg
Catalase = + Capnophilic = +
“Gliding” on the agar was noted.
What is the most likely identification?
A. Pseudomonas aeruginosa
B. Capnocytophaga canimorsus
C. Acinetobacter baumannii
D. Proteus mirabilis

Answer 13

B. Capnocytophaga canimorsus

Note: 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.

Question 14

A patient exhibits fever, chills, abdominal cramps, diarrhea, vomiting, and bloody stools 10 to 12 hours after eating. Which organisms will most likely grow from this patient’s stool culture?
A. Salmonella or Yersinia spp.
B. E. coli O157:H7 or Shigella spp.
C. Staphylococcus aureus or Clostridium perfringens
D. Salmonella or Staphylococcus spp.

Answer 14

B. E. coli O157:H7 or Shigella spp.

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

Question 15

When testing for coagulase properties, staphylococci isolates from a 67-year-old male diabetic patient showed a positive tube test (free
coagulase). The organism should be identified as:
A. Staphylococcus aureus
B. Staphylococcus haemolyticus
C. Staphylococcus saprophyticus
D. Micrococcus luteus

Answer 15

A. Staphylococcus aureus

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

Question 16

An isolate of Staphylococcus aureus was cultured from an ulcer obtained from the leg of a diabetic 79-year-old female patient. The organism showed
resistance to methicillin. Additionally, this isolate should be tested for resistance or susceptibility to:
A. Erythromycin
B. Gentamicin
C. Vancomycin
D. Kanamycin

Answer 16

C. Vancomycin

Note: MRSA isolates are usually tested for susceptibility or resistance to vancomycin, a glycopeptide.

Question 17

An isolate recovered from a vaginal culture obtained from a 25-year-old female patient who is 8 months pregnant is shown to be a gram-positive cocci, catalase negative, and β-hemolytic on blood agar. Which tests are needed for further identification?
A. Optochin, bile solubility, PYR
B. Bacitracin, CAMP, PYR
C. Methicillin, PYR, trehalose
D. Coagulase, glucose, PYR

Answer 17

B. Bacitracin, CAMP, PYR

Note: 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.

Question 18

Which organism is the most often recovered gram-positive cocci (catalase negative) from a series of
blood cultures obtained from individuals with endocarditis?
A. Streptococcus agalactiae
B. Clostridium perfringens
C. Enterococcus faecalis
D. Pediococcus spp.

Answer 18

C. Enterococcus faecalis

Note: 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.

Question 19

A presumptive diagnosis of gonorrhea can be made from an exudate from a 20-year-old emergency department patient if which of the following criteria are present?
A. Smear of urethral exudate (male only) shows typical gram-negative, intracellular diplococci; growth of oxidase-positive, gram-negative diplococci on selective agar (modified Thayer–Martin)
B. Smear from vaginal area shows gram-negative diplococci; growth of typical colonies on blood agar
C. Smear from rectum shows typical gram-negative diplococci; no growth on chocolate agar
D. Growth of gram-negative cocci on MacConkey agar and blood agar

Answer 19

A. Smear of urethral exudate (male only) shows typical gram-negative, intracellular diplococci; growth of oxidase-positive, gram-negative diplococci on selective agar (modified Thayer–Martin)

Note: N. gonorrhoeae can be presumptively identified from a male patient only from the Gram stain 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).

Question 20

“Clue cells” are seen on a smear of vaginal discharge obtained from an 18-year-old female emergency department patient. This finding,
along with a fishy odor (amine) after the addition of 10% KOH, suggests bacterial vaginosis caused by which organism?
A. Staphylococcus epidermidis
B. Streptococcus agalactiae
C. Gardnerella vaginalis
D. E. coli

Answer 20

C. Gardnerella vaginalis

Note: 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.

Question 21

A 1-month-old infant underwent a spinal tap to rule out bacterial meningitis. The CSF was cloudy, and the smear showed many pus cells and
short gram-positive rods. After 18 hr, many colonies appeared on blood agar that resembled Streptococcus spp. or L. monocytogenes. Which
of the following preliminary tests should be performed on the colonies to best differentiate L. monocytogenes from Streptococcus spp.?
A. Hanging-drop motility (25°C) and catalase
B. PYR and bacitracin
C. Oxidase and glucose
D. Coagulase and catalase

Answer 21

A. Hanging-drop motility (25°C) and catalase

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

Question 22

Acid-fast positive bacilli were recovered from the sputum of a 79-year-old man who had been treated for pneumonia. Which of the following
test reactions after 3 weeks of incubation on Löwenstein–Jensen agar are consistent with Mycobacterium tuberculosis?
A. Niacin = + Nitrate reduction = + Photochromogenic = Neg
B. Niacin= Neg Optochin = +
Catalase = +
C. PYR = + Urease = + Bacitracin = +
D. Ampicillin = Resistant Penicillin = Resistant

Answer 22

A. Niacin = + Nitrate reduction = + Photochromogenic = Neg

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

Question 23

Which biochemical tests should be performed in order to identify colorless colonies growing on MacConkey agar (swarming colonies on blood agar) from a catheterized urine specimen?
A. Indole, phenylalanine deaminase, and urease
B. Glucose, oxidase, and lactose utilization
C. Phenylalanine deaminase and bile solubility
D. H2S and catalase

Answer 23

A. Indole, phenylalanine deaminase, and urease

Note: A swarmer on blood agar would most likely be a Proteus spp. A lactose nonfermenter and swarmer that is often isolated from urinary tract infections is P. mirabilis.

Question 24

A gram-negative nonfermenter was isolated from a culture taken from a burn patient. Which of the following is the best choice of tests to differentiate Pseudomonas aeruginosa from Acinetobacter spp.?
A. Growth on MacConkey agar, catalase, growth at 37°C
B. Oxidase, motility, growth at 42°C
C. Growth on blood agar, oxidase, growth at 35°C
D. String test and coagulase test

Answer 24

B. Oxidase, motility, growth at 42°C

Note: P. aeruginosa has a distinctive grape odor. The best choice of tests is:
42° C Growth, Oxidase, Motility
P. aeruginosa + + +
Acinetobacter spp. +/Neg Neg Neg

Question 25

A Haemophilus spp., recovered from a throat culture obtained from a 59-year-old male patient undergoing chemotherapy, required hemin (X factor) and NAD (V factor) for growth. This species also hemolyzed horse
erythrocytes on blood agar. What is the most likely species?
A. H. ducreyi
B. H. parahaemolyticus
C. H. haemolyticus
D. H. aegyptius

Answer 25

C. H. haemolyticus

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

Question 26

Large gram-positive bacilli (boxcar shaped) were recovered from a blood culture taken from a 70-year-old female diabetic patient. The following
results were recorded:
Aerobic growth = Neg
Anaerobic growth = +
Spores = Neg Motility = Neg
Lecithinase = +
Hemolysis = β (double zone)
GLC (volatile acids) = acetic acid and butyric acid
What is the most likely identification?
A. Clostridium perfringens
B. Fusobacterium spp.
C. Bacteroides spp.
D. Clostridium sporogenes

Answer 26

A. Clostridium perfringens

Note: 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.

Question 27

Anaerobic gram-negative rods were recovered from the blood of a patient after gallbladder surgery. The bacteria grew well on agar containing 20% bile, but were resistant to kanamycin and
vancomycin. What is the most likely
identification?
A. Clostridium perfringens
B. Bacteroides fragilis group
C. Prevotella spp.
D. Porphyromonas spp.

Answer 27

B. Bacteroides fragilis group

Note: B. fragilis is the most often 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.

Question 28

In Breakpoint Antimicrobial Drug Testing, interpretation of susceptible (S), intermediate (I), and resistant (R) refers to testing antibiotics by using:
A. The amount needed to cause bacteriostasis
B. Only the specific concentrations necessary to report S, I, or R
C. An MIC of 64 μg/mL
D. A dilution of drug that is one tube less than the toxic level

Answer 28

B. Only the specific concentrations necessary to report S, I, or R

Note: 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.

Question 29

A CSF sample obtained from a 2-week old infant with suspected bacterial meningitis grew gram-negative rods on blood and chocolate agars.
The following results were noted:
MacConkey agar = No growth
ONPG = + Glucose (open) OF = + Urease = Neg Indole = + Oxidase = +
Glucose (closed) OF = Neg Catalase = +
Motility = Neg Pigment = Yellow
42° C growth = Neg
What is the correct identification?
A. Pseudomonas aeruginosa
B. Chryseobacterium meningosepticum
C. Acinetobacter baumannii
D. E. coli

Answer 29

B. Chryseobacterium meningosepticum

Note: C. meningosepticum is a well-known cause of neonatal meningitis. It will grow well on chocolate agar, producing yellow pigmented colonies.

Question 30

During the summer break, several middle-aged elementary school teachers from the same school
district attended a 3-day seminar in Chicago. Upon returning home, three female teachers from the group were hospitalized with pneumonia, flulike symptoms, and a nonproductive cough. Routine testing of sputum samples revealed normal flora. Further testing using buffered CYE agar with L-cysteine and α-ketoglutarate in 5% CO2 produced growth of opaque colonies that stained faintly, showing thin gram-negative rods. What is the most likely identification?
A. Legionella pneumophila
B. Haemophilus influenzae
C. Eikenella corrodens
D. Streptococcus pneumoniae

Answer 30

A. Legionella pneumophila

Note: L. pneumophila is the cause of pneumonia and can occur as part of an epidemic sporadically or nosocomially, or may be community acquired. The appearance of mottled, cut-glass colonies on buffered CYE agar under low power and the use of a direct immunofluorescence 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.

Question 31

A vancomycin-resistant gram-positive
coccobacillus resembling the Streptococcus viridans group was isolated from the blood of a 42-year-old female patient undergoing a bone
marrow transplant. The PYR and leucine aminopeptidase (LAP) tests were negative. The following results were noted:
Catalase = Neg CAMP = Neg
Esculin hydrolysis = Neg
Gas from glucose = +
Hippurate hydrolysis = Neg
6.5% salt broth = Neg
What is the correct identification?
A. Leuconostoc spp.
B. Enterococcus spp.
C. Staphylococcus spp.
D. Micrococcus spp.

Answer 31

A. Leuconostoc spp.

Note: Leuconostoc spp. are vancomycin-resistant opportunistic pathogens and follow invasive procedures. They are often recovered from positive neonatal blood cultures resulting from colonization during delivery.

Question 32

A catalase-negative, gram-positive coccus resembling staphylococci (clusters on the Gram-stained smear) was recovered from three different
blood cultures obtained from a 60-year-old patient diagnosed with endocarditis. The following test
results were noted:
PYR = Neg LAP = Neg (V)
Esculin hydrolysis = Neg
6.5% Salt broth = Neg
Vancomycin = Sensitive
CAMP test = Neg
What is the correct identification?
A. Leuconostoc spp.
B. Gemella spp.
C. Enterococcus spp.
D. Micrococcus spp.

Answer 32

B. Gemella spp.

Note: Gemella spp. are often recovered from patients with endocarditis and meningitis. On the Gram stain, they resemble staphylococci morphologically but
are catalase negative.

Question 33

An immunocompromised patient with prior antibiotic treatment grew aerobic gram-positive cocci from several clinical specimens that were
cultured. The organism was vancomycin resistant and catalase negative. Additional testing proved
negative for enterococci. What other groups of organisms might be responsible?
A. Leuconostoc spp. and Pediococcus spp.
B. Streptococcus pyogenes and Streptococcus agalactiae
C. Micrococcus spp. and Gemella spp.
D. Clostridium spp. and Streptococcus bovis

Answer 33

A. Leuconostoc spp. and Pediococcus spp.

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

Question 34

A catalase-positive, gram-positive coccus (clusters on Gram stain smear) grew pale yellow, creamy colonies on 5% sheep blood agar. The specimen
was recovered from pustules on the face of a 5-year-old girl with impetigo. The following test reactions indicate which organism?
Glucose = + (Fermentation)
Oxidase = Neg
PYR = Neg Bacitracin = Sensitive
Lysostaphin = Sensitive
A. Micrococcus spp.
B. Streptococcus spp.
C. Enterococcus spp.
D. Staphylococcus spp.

Answer 34

D. Staphylococcus spp.

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

Question 35

A wound (skin lesion) specimen obtained from a newborn grew predominantly β-hemolytic colonies of gram-positive cocci on 5% sheep blood agar. The newborn infant was covered with small skin eruptions that gave the appearance of a “scalding of the skin.” The gram-positive cocci
proved to be catalase positive. Which tests should follow for the appropriate identification?
A. Optochin, bile solubility, PYR
B. Coagulase, glucose fermentation, DNase
C. Bacitracin, PYR, 6.5% salt broth
D. CAMP, bile-esculin, 6.5% salt broth

Answer 35

B. Coagulase, glucose fermentation, DNase

Note: 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.

Question 36

A 20-year-old female patient entered the emergency clinic complaining of abdominal pain, fever, and a burning sensation during urination. An above-normal WBC count along with pus cells and bacteria in the urine specimen prompted the emergency physician to order a urine culture. The
colony count reported for this patient revealed >100,000 col/mL of a nonhemolytic, catalase negative,
gram-positive organism on 5% sheep
blood agar. The following test results indicate which organism?
PYR = + Bile Esculin = +
6.5% Salt broth = + growth
Bacitracin = Neg Optochin = Neg
A. Enterococcus faecalis
B. Streptococcus pyogenes
C. Streptococcus agalactiae
D. Streptococcus bovis

Answer 36

A. Enterococcus faecalis

Note: E. faecalis gives a positive reaction to the PYR test and is often implicated in urinary tract infections
(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.

Question 37

A sputum specimen from an 89-year-old male patient with suspected bacterial pneumonia grew a predominance of gram-positive cocci displaying alpha-hemolysis on 5% sheep blood agar. The colonies appeared donut shaped and mucoidy and tested negative for catalase. The most appropriate tests for a final identification are:
A. Coagulase, glucose fermentation, lysostaphin
B. Penicillin, bacitracin, CAMP
C. Optochin, bile solubility, PYR
D. Bile esculin, hippurate hydrolysis

Answer 37

C. Optochin, bile solubility, PYR

Note: S. pneumoniae colonies appear as α-hemolytic “donut” shaped colonies on 5% sheep blood agar.
The mucoid colonies may appear “wet” or “watery” due to the capsule surrounding the organism. The Gram-stained 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.

Question 38

A tissue biopsy specimen of the stomach was obtained from a 38-year-old male patient diagnosed with gastric ulcers. The specimen was transported immediately and processed for culture and histology. At 5 days, the culture produced colonies of gram-negative (curved)
bacilli on chocolate and Brucella agar with 5% sheep blood. The cultures were held at 35°C–37°C in a microaerophilic atmosphere. The colonies tested positive for urease. The most likely identification is:
A. E. coli
B. Helicobacter pylori
C. Enterococcus faecalis
D. Streptococcus bovis

Answer 38

B. Helicobacter pylori

Note: 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.

Question 39

A catalase-positive, gram-positive short rod was recovered from the blood of a prenatal patient. The organism appeared on 5% sheep blood as white colonies surrounded by a small zone of beta-hemolysis. The following tests were performed, indicating the patient was infected
with which organism?
Motility = + (tumbling on wet prep) room temperature
Motility = + (umbrella-shape on semisolid agar)
room temperature
Glucose = + (fermentation)
Esculin = +
Voges–Proskauer = +
A. Listeria monocytogenes
B. Streptococcus agalactiae
C. Streptococcus pyogenes
D. Lactobacillus spp.

Answer 39

A. Listeria monocytogenes

Note: 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 a
Gram stain will differentiate the two organisms. L. monocytogenes are catalase positive and are motile
(tumbling motility at room temperature) as well as rod shaped instead of cocci shaped.

Question 40

An emergency department physician suspected Corynebacterium diphtheriae when examining the
sore throat of an exchange student from South America. What is the appropriate media for the culture of the nasopharyngeal swab obtained
from the patient?
A. Chocolate agar
B. Thayer–Martin agar
C. Tinsdale medium
D. MacConkey agar

Answer 40

C. Tinsdale medium

Note: 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 places for exposure to C. diphtheriae. 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.

Question 41

A 25-year-old pregnant patient complained of vaginal irritation. Cultures taken for STDs proved
negative. A Gram-stained vaginal smear revealed many epithelial cells with gram-variable short rods (coccobacilli) covering the margins. What is the most likely cause of the vaginosis?
A. Group B streptococci spp.
B. Gardnerella vaginalis
C. Staphylococcus aureus
D. Staphylococcus saprophyticus

Answer 41

B. Gardnerella vaginalis

Note: 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.

Question 42

A 50-year-old male transplant patient was experiencing neurological difficulties after a pulmonary infection. A spinal tap revealed a
cloudy CSF with a Gram-stained smear revealing gram-positive long-beaded bacilli. An acid-fast smear showed filamentous partially acid-fast bacilli. What is the most likely identification of the organism?
A. Nocardia asteroides
B. Mycobacterium avium
C. Mycobacterium bovis
D. Legionella spp.

Answer 42

A. Nocardia asteroides

Note: 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, which then disseminate to other organs, often proving fatal.

Question 43

A 22-year-old pregnant woman (third trimester) entered the emergency department complaining of diarrhea, fever, and other flulike symptoms.
Blood cultures were ordered along with a urine culture. After 24 hours, the urine culture was negative, but the blood cultures revealed a gram-positive short rod that grew aerobically on blood agar. The colonies were small and smooth,
resembling a Streptococcus spp. with a small narrow zone of β-hemolysis. The following test results indicate which organism?
Motility = + (Wet mount = Tumbling)
Glucose = + (Acid) Catalase = +
Esculin hydrolysis = +
A. Listeria monocytogenes
B. Streptococcus pneumoniae
C. Streptococcus agalactiae
D. Corynebacterium spp.

Answer 43

A. Listeria monocytogenes

Note: 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.

Question 44

Anaerobic gram-positive, spore-forming bacilli were recovered from the feces of a chemotherapy patient with severe diarrhea. The patient had
undergone antibiotic therapy 1 week prior. The fecal culture produced growth only on the CCFA plate. No aerobic growth of normal flora was seen after 48 hours. The following results were noted:
Kanamycin = Sensitive
Vancomycin = Sensitive
Colistin = Resistant Lecithinase = Neg
Lipase = Neg Nitrate = Neg
Indole = Neg Urease = Neg
Catalase = Neg Spores = +
CCFA agar = Growth of yellow, “ground-glass” colonies that fluoresce chartreuse (yellow-green)
What is the correct identification?
A. Clostridium perfringens
B. Clostridium tetani
C. Clostridium sordellii
D. Clostridium difficile

Answer 44

D. Clostridium difficile

Note: The overgrowth of C. difficile in the bowel is the cause of antimicrobial-associated colitis. Culturing for C. difficile is the least specific but the most sensitive
method to detect possible disease related to C. difficile. A characteristic “horse-stable” odor is noted on CCFA growing C. difficile.

Question 45

Anaerobic gram-positive diphtheroids (nonspore formers) were cultured from two separate blood culture bottles (at 5 days) obtained from a
25-year-old patient admitted to the hospital with dehydration, diarrhea, and other flulike symptoms. Four other blood culture bottles did not grow any organisms at 7 days and were discarded. The following results were obtained from the recovered anaerobe:
Indole = + Nitrate = + Catalase = +
Kanamycin = Sensitive
Vancomycin = Sensitive
Colistin = Resistant
Major acid from PYG broth by GLC = Propionic acid
What is the correct identification?
A. Eubacterium lentum
B. Propionibacterium acnes
C. Actinomyces spp.
D. Peptostreptococcus spp.

Answer 45

B. Propionibacterium acnes

Note: P. acnes is a diphtheroid (pleomorphic rod) that may appear to branch on the Gram-stained 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.

Question 46

Anaerobic gram-positive bacilli with subterminal spores were recovered from several blood cultures obtained from a patient diagnosed with a
malignancy of the colon. The following results were recorded:
Indole = Neg Urease = Neg
Lipase = Neg Catalase = Neg Lecithinase = Neg Growth on blood agar = Swarming colonies
What is the correct identification?
A. Clostridium septicum
B. Clostridium perfringens
C. Clostridium sordellii
D. Propionibacterium acnes

Answer 46

A. Clostridium septicum

Note: 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.
Indole, Urease, Spores
C. septicum: Neg Neg Subterminal
C. tetani: –/+ Neg Terminal
C. sordellii: + + Subterminal

Question 47

Anaerobic gram-negative bacilli were recovered from fluid obtained from drainage of a postsurgical abdominal wound. The following test results were recorded:
Kanamycin = Resistant
Vancomycin = Resistant
Colistin = Resistant
Growth on 20% bile plate = +
Pigment = Neg
Indole = V (Neg) Nitrate = Neg
Urease = Neg Lipase = Neg
What is the correct identification?
A. Prevotella spp.
B. Bacteroides fragilis group
C. Porphyromonas spp.
D. Clostridium spp.

Answer 47

B. Bacteroides fragilis group

Note: 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.

Question 48

Anaerobic, nonpigmented, gram-negative rods were recovered from an anaerobic blood agar plate after 48 hours of incubation. The Gram-stained smear showed thin bacilli with pointed ends. The colonies on blood agar had the appearance of dry,
irregular, white breadcrumb-like morphology with greening of the agar. The following reactions were
noted:
Kanamycin = Sensitive
Vancomycin = Resistant
Colistin = Sensitive Nitrate = Neg
Indole = + Catalase = Neg
Lipase = Neg Urease = Neg
Growth on 20% bile agar = Neg
What is the correct identification?
A. Fusobacterium nucleatum
B. Bacteroides fragilis
C. Clostridium perfringens
D. Peptostreptococcus spp.

Answer 48

A. Fusobacterium nucleatum

Note: 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.

Question 49

A 2-month-old infant in good health was scheduled for a checkup at the pediatrician’s office. After arriving for the appointment, the mother noted white patches on the baby’s tongue
and in his mouth. The baby constantly used a pacifier. What is the most likely organism causing the white patches?
A. Cryptococcus neoformans
B. Candida albicans
C. Aspergillus fumigatus
D. None of these options

Answer 49

B. Candida albicans

Note: 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 the skin, mucous membranes, and gastrointestinal tract.

Question 50

A 69-year-old male patient who was a cigarette smoker visited the doctor’s office complaining of a cough and congestion of the lungs. Routine
cultures of early morning sputum (×3) for bacteria as well as for AFB revealed no pathogens. A fungal culture was also ordered that grew the following on Sabouraud dextrose agar after 3 days:
Hyphae = Septate with dichotomous branching
Spores = Produced by conidial heads with numerous conidia
Colonies = Velvety or powdery, white at first, then turning dark greenish to gray (reverse = white to tan)
Vesicle = Holding phialides usually on upper two-thirds only
What is the most likely identification?
A. Aspergillus niger
B. Absidia spp.
C. Mucor spp.
D. Aspergillus fumigatus

Answer 50

D. Aspergillus fumigatus

Note: 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 neutropenic patients exhibits sinusitis, and is disseminated throughout the body.

Question 51

A young male patient with a fungus of the feet visited the podiatrist for relief from the itching. A culture was sent to the microbiology laboratory that grew after 8 days on Sabouraud dextrose
agar. Colonies were powdery pink with concentric and radial folds, with the reverse side showing brownish-tan to red in color. Other observations
were:
Hyphae = Septate Urease = +
Macroconidia = Cigar shaped, thin walled with 1–6 cells
Microconidia = Round and clustered on branched conidiophores
Red pigment on cornmeal (1% dextrose) = Neg
In vitro hair perforation = +
The most likely identification is:
A. Trichophyton mentagrophytes
B. Trichophyton rubrum
C. Candida albicans
D. Aspergillus niger

Answer 51

A. Trichophyton mentagrophytes

Note: T. mentagrophytes, a common cause of athletes’ foot, is sometimes confused with T. rubrum, the most
common dermatophyte to infect humans.

Question 52

A 79-year-old female nursing home patient was admitted to the hospital with a fever and central nervous system dysfunction. Routine blood work and blood cultures were ordered. After 48 hours, the blood cultures revealed a budding yeast. The
following tests performed from Sabouraud dextrose agar (after 3 days of growth) showed:
Germ tube = Neg
growth Birdseed agar = Brown
Urease = + Pseudohyphae = Neg
Blastospores =+
Chlamydospores = Neg
Arthrospores = Neg
Assimilation agar = + (dextrose, sucrose, maltose)
What is the most likely identification?
A. Candida albicans
B. Cryptococcus laurentii
C. Cryptococcus neoformans
D. Candida tropicalis

Answer 52

C. Cryptococcus neoformans

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

Question 53

A dehydrated 25-year-old male patient was admitted to the hospital with symptoms similar to those of chronic fatigue syndrome. Serological testing proved negative for recent streptococcal infection, Epstein–Barr virus, and hepatitis. Which of the following viral serological tests
should help with a possible diagnosis?
A. CMV
B. Echovirus
C. Respiratory syncytial virus
D. Measles virus

Answer 53

A. CMV

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

Question 54

A nursing student working in the emergency department accidentally stuck herself with a needle after removing it from an intravenous set taken from a suspected drug user. The best course of action, after reporting the incident to her supervisor, is to:
A. Test the student for HIV virus if flulike symptoms develop in 2–4 weeks
B. Immediately test the patient and the student for HIV using an EIA or ELISA test
C. Perform a Western blot assay on the student’s serum
D. Draw blood from the student only and freeze it for further testing

Answer 54

B. Immediately test the patient and the student for HIV using an EIA or ELISA test

Note: With the permission of the patient (state law may require him or her to sign a consent form) and
counseling of the student nurse, the appropriate course of action is to test the patient for HIV using a screening test (EIA or ELISA). The student should
also be baseline tested. If the 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 is done
on any positive HIV tests.

Question 55

A 30-year-old female patient complained of vaginal irritation and symptoms (fever, dysuria, and inguinal lymphadenopathy) associated with sexually transmitted disease (STD). Examination showed extensive lesions in the genital area. Chlamydia spp. testing, Neisseria gonorrhoeae,
and Gardnerella vaginalis cultures were negative. Rapid plasma reagin (RPR) testing was also negative. What is the next line of testing?
A. Darkfield examination
B. Herpes simplex testing
C. Trichomonas spp. testing
D. Group B streptococcal testing

Answer 55

B. Herpes simplex testing

Note: 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.

Question 56

A patient is being seen in the emergency department for a low-grade fever, headache, and general malaise after returning from Africa on a photographic safari. The physician has requested blood for malaria; the laboratory would like to have patient information regarding:
A. Specific travel history and body temperature every 4 hours
B. Liver function tests and prophylactic medication history
C. Transfusion history and body temperature every 4 hours
D. Prophylactic medication history and specific travel history

Answer 56

D. Prophylactic medication history and specific travel history

Note: 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. Also specific geographic
travel history may help to determine whether chloroquine-resistant Plasmodium falciparum may be a factor.

Question 57

Examination of a modified acid-fast stained fecal smear reveals round structures measuring approximately 8–10 μm, some of which are stained and some of which are not. They do not appear to show any internal morphology. The patient is symptomatic with diarrhea, and the
cause may be:
A. Blastocystis hominis
B. Polymorphonuclear leukocytes
C. Cyclospora cayetanensis
D. Large yeast cells

Answer 57

C. Cyclospora cayetanensis

Note: One of the newer 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 oocysts are immature when passed (no internal morphology) and they measure about 8–10 μm.

Question 58

A patient has been diagnosed as having amebiasis but continues to be asymptomatic. The physician has asked for an explanation and recommendations regarding follow-up. Suggestions should include:
A. Consideration of Entamoeba histolytica versus Entamoeba dispar
B. A request for an additional three stools for culture
C. Initiating therapy, regardless of the patient’s asymptomatic status
D. Performance of barium x-ray studies

Answer 58

A. Consideration of Entamoeba histolytica versus Entamoeba dispar

Note: It is now well established that E. histolytica is being used to designate the pathogenic species, whereas E. dispar is now being used to designate
the nonpathogenic species. However, unless trophozoites containing ingested red blood cells (E. histolytica) are seen, the two organisms cannot
be differentiated on the basis of morphology. Based on 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/ E. dispar—unable to differentiate on the basis of
morphology unless trophozoites are seen to contain ingested RBCs (E. histolytica).”

Question 59

Although a patient is strongly suspected of having giardiasis and is still symptomatic, three routine
stool examinations (O&P exam) have been performed correctly and reported as negative. Biopsy confirmed the patient had giardiasis.
Reasons for these findings may include:
A. The patient was coinfected with several bacterial species
B. Giardia lamblia tends to adhere to the mucosal surface and more than three stool examinations may be required to confirm a suspected infection
C. The organisms present did not stain with trichrome stain and therefore the morphology is very atypical
D. Special diagnostic procedures such as the Knott concentration and nutrient-free agar cultures should have been used

Answer 59

B. Giardia lamblia tends to adhere to the mucosal surface and more than three stool examinations may be required to confirm a suspected infection

Note: It is well known that G. lamblia 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. If fecal immunoassays are used, two different stool specimens should be tested before indicating
the patient is negative.

Question 60

A transplant patient is currently receiving steroids. The patient is now complaining of abdominal pain and has symptoms of pneumonia and positive blood cultures with gram-negative rods. The individual has been living in the United States for 20 years but grew up in Central America. The most likely parasite causing these symptoms is:
A. Trypanosoma brucei rhodesiense
B. Giardia lamblia
C. Strongyloides stercoralis
D. Schistosoma japonicum

Answer 60

C. Strongyloides stercoralis

Note: 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). Patients who become immunosuppressed may see the life cycle of Strongyloides reactivated
with serious illness resulting; this can occur many years after the initial infection and after the patient has left the endemic area.