Gram-Positive and Gram-Negative Cocci

Question 1

The test used most often to separate the Micrococcaceae family from the Streptococcaceae family is:
A. Bacitracin
B. Catalase
C. Hemolysis pattern
D. All of these options

Answer 1

B. Catalase

Note: The catalase test (utilizing a 3% hydrogen peroxide [H2O2] solution stored in a brown bottle under refrigeration) is positive for the four genera belonging to the Micrococcaceae family: Planococcus, Micrococcus, Stomatococcus, and Staphylococcus. Members of the Streptococcaceae family are negative. Planococcus spp. are associated with marine life and not human infections. Stomatococcus spp. are implicated in endocarditis following cardiac catheterization; they are weakly catalase positive and produce white or transparent sticky colonies on agar, which help to differentiate them from Staphylococcus.

Question 2

Micrococcus and Staphylococcus species are differentiated by which test(s)?
A. Fermentation of glucose (OF tube)
B. Catalase test
C. Gram stain
D. All of these options

Answer 2

A. Fermentation of glucose (OF tube)

Note: 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.

Question 3

Lysostaphin is used to differentiate Staphylococcus from which other genus?
A. Streptococcus
B. Stomatococcus
C. Micrococcus
D. Planococcus

Answer 3

C. Micrococcus

Note: Lysostaphin is an endopeptidase that cleaves the glycine-rich pentapeptide crossbridges in the staphylococcal cell wall peptidoglycan. The susceptibility of the staphylococci to lysostaphin is used to differentiate them from the micrococci. Staphylococci are susceptible and show a 10-16 mm zone of inhibition, while micrococci are not inhibited.

Question 4

Which of the following tests is used routinely to identify Staphylococcus aureus?
A. Slide coagulase test
B. Tube coagulase test
C. Latex agglutination
D. All of these options

Answer 4

D. All of these options

Note: 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 results must be confirmed by the tube method for detecting “free coagulase” or extracellular coagulase. The tube test is usually positive within 4 hours at 35C; however, a negative result must then be incubated at room temperature for the remainder of 18-24 hours. Some strains produce coagulase slowly or produce fibrinolysin, which dissolves the clot at 35°C. Latex agglutination procedures utilize fibrinogen and IgG-coated latex beads that detect protein A on the staphylococcal cell wall.

Question 5

Which of the following enzymes contribute to the
virulence of S. aureus?
A. Urease and lecithinase
B. Hyaluronidase and β-lactamase
C. Lecithinase and catalase
D. Cytochrome oxidase

Answer 5

B. Hyaluronidase and B-lactamase

Note: 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.

Question 6

Toxic shock syndrome is attributed to infection with:
A. Staphylococcus epidermidis
B. Staphylococcus hominis
C. Staphylococcus aureus
D. Staphylococcus saprophyticus

Answer 6

C. Staphylococcus aureus

Note: 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 life-threatening systemic damage as well as shock.

Question 7

Which Staphylococcus species, in addition to S. aureus, also produces coagulase?
A. S. intermedius
B. S. saprophyticus
C. S. hominis
D. All of these options

Answer 7

A. S. intermedius

Note: S. intermedius 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.

Question 8

Staphylococcus epidermidis (coagulase negative) is recovered from which of the following sources?
A. Prosthetic heart valves
B. Intravenous catheters
C. Urinary tract
D. All of these options

Answer 8

D. All of these options

Note: S. epidermidis represents 50-80% 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.

Question 9

Slime production is associated with which Staphylococcus species?
A. S. aureus
B. S. epidermidis
C. S. intermedius
D. S. saprophyticus

Answer 9

B. S. epidermidis

Note: S. epidermidis produces an extracellular slime 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.

Question 10

Strains of Staphylococcus species resistant to the β-lactam antibiotics by standardized disk diffusion and broth microdilution susceptibility methods
are called:
A. Heteroresistant
B. Bacteriophage group 52A
C. Cross resistant
D. Plasmid altered

Answer 10

A. Heteroresistant

Note: 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%–4% NaCl, and incubation up to 48 hours.

Question 11

Staphylococcus saprophyticus is best differentiated from Staphylococcus epidermidis by resistance to:
A. 5 μg of lysostaphin
B. 5 μg of novobiocin
C. 10 units of penicillin
D. 0.04 unit of bacitracin

Answer 11

B. 5 Ug of novobiocin

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

Question 12

The following results were observed by using a tube coagulase test:
Coagulase at 4 hours = +
Coagulase at 18 hours = Neg
DNase = + Novobiocin = Sensitive
(16-mm zone)
Hemolysis on blood Mannitol salt plate = +
agar = β (acid production)
What is the most probable identification?
A. Staphylococcus saprophyticus
B. Staphylococcus epidermidis
C. Staphylococcus aureus
D. Staphylococcus hominis

Answer 12

C. Staphylococcus aureus

Note: 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–37°C and 18–24 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. aureus.

Question 13

Staphylococcus aureus recovered from a wound culture gave the following antibiotic sensitivity pattern by the standardized Kirby–Bauer method (S = sensitive; R = resistant):
Penicillin = R Ampicillin = S
Cephalothin = R Cefoxitin = R
Vancomycin = S Methicillin = R
Which is the drug of choice for treating this infection?
A. Penicillin
B. Ampicillin
C. Cephalothin
D. Vancomycin

Answer 13

D. Vancomycin

Note: Vancomycin, along with rifampin, is used for strains of S. aureus that are resistant to the B-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.

Question 14

Which of the following tests should be used to differentiate Staphylococcus aureus from Staphylococcus intermedius?
A. Acetoin
B. Catalase
C. Slide coagulase test
D. Urease

Answer 14

A. Acetoin

Note: The production of acetoin by S. aureus from glucose or pyruvate differentiates it from S. intermedius, which is also coagulase positive. This test is also called the VP test. Acetoin production is detected by addition of 40% KOH and 1% alpha-naphthol to the VP test broth after 48 hours of incubation. A distinct pink color within 10 minutes denotes a positive test.

Question 14

A gram-positive coccus recovered from a wound ulcer from a 31-year-old diabetic patient showed pale yellow, creamy, β-hemolytic colonies on blood agar. Given the following test results, what is the most likely identification?
Catalase = +
Glucose OF: positive open tube, negative sealed tube
Mannitol salt = Neg
Slide coagulase = Neg
A. Staphylococcus aureus
B. Staphylococcus epidermidis
C. Micrococcus spp.
D. Streptococcus spp.

Answer 14

C. Micrococcus spp.

Note: 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).

Question 15

Urine cultured from the catheter of an 18-year-old female patient produced more than 100,000 col/mL on a CNA plate. Colonies were catalase positive, coagulase negative by the latex agglutination slide method as well as the tube coagulase test. The best single test for identification is:
A. Lactose fermentation
B. Urease
C. Catalase
D. Novobiocin susceptibility

Answer 15

D. Novobiocin susceptibility

Note: 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 ug of novobiocin.

Question 16

A Staphylococcus spp. recovered from a wound (cellulitis) was negative for the slide coagulase test (clumping factor) and negative for novobiocin
resistance. The next test(s) needed for
identification is (are):
A. Tube coagulase test
B. β-Hemolysis on blood agar
C. Mannitol salt agar plate
D. All of these options

Answer 16

D. All of these options

Note: S. aureus is novobiocin sensitive and cannot be ruled out by a negative clumping factor test. Most S. aureus produce B-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 should be performed because the slide test was negative.

Question 17

Furazolidone (Furoxone) susceptibility is a test used to differentiate:

A. Staphylococcus spp. from Micrococcus spp.
B. Streptococcus spp. from Staphylococcus spp.
C. Staphylococcus spp. from Pseudomonas spp.
D. Streptococcus spp. from Micrococcus spp.

Answer 17

A. Staphylococcus spp. from Micrococcus spp.

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

Question 18

Bacitracin resistance (0.04 unit) is used to differentiate:
A. Micrococcus spp. from Staphylococcus spp.
B. Staphylococcus spp. from Neisseria spp.
C. Planococcus spp. from Micrococcus spp.
D. Staphylococcus spp. from Streptococcus spp.

Answer 18

A. Micrococcus spp. from Staphylococcus spp.

Note: A bacitracin disk (0.04 unit) is used to identify group A B-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, while Micrococcus species are sensitive.

Question 19

Which of the following tests will rapidly differentiate micrococci from staphylococci?
A. Catalase
B. Coagulase
C. Modified oxidase
D. Novobiocin susceptibility

Answer 19

C. Modified oxidase

Note: The modified oxidase test 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-p-phenylenediamine in dimethylsulfoxide) are used. A colony of the isolate is rubbed onto the paper. Oxidase-positive organisms produce a purple color within 30 sec.

Question 20

Streptococcus species exhibit which of the following properties?
A. Aerobic, oxidase positive, and catalase positive
B. Facultative anaerobe, oxidase negative, catalase negative
C. Facultative anaerobe, β-hemolytic, catalase positive
D. May be α-, β-, or γ-hemolytic, catalase positive

Answer 20

B. Facultative anaerobe, oxidase negative, catalase negative

Note: Streptococcus species are facultative anaerobes that grow aerobically as well, and are oxidase and catalase negative. In order to demonstrate streptolysin O on blood agar, it is best to stab the agar to create anaerobiois because streptolysin O is oxygen labile.

Question 21

Which group of streptococci is associated with erythrogenic toxin production?
A. Group A
B. Group B
C. Group C
D. Group G

Answer 21

A. Group A

Note: Group A Beta-hemolytic streptococci are the cause of scarlet fever, and some strains produce toxins (pyrogenic exotoxins A, B, and C) that cause a scarlatiniform rash.

Question 22

A fourfold rise in titer of which antibody is the best indicator of a recent infection with group A β-hemolytic streptococci?
A. Anti-streptolysin O
B. Anti-streptolysin S
C. Anti-A
D. Anti-B

Answer 22

A. Anti-streptolysin O

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

Question 23

Bacitracin A disks (0.04 unit) are used for the presumptive identification of which group of Beta-hemolytic streptococci?
A. Group A
B. Group B
C. Group C
D. Group F

Answer 23

A. Group A

Note: The bacitracin disk test is used in conjunction with other confirmatory tests for the Beta-hemolytic streptococci. In addition to group A, groups C, F, and G are also Beta-hemolytic and give a positive test for bacitracin (a zone of inhibition of any size). Therefore, a positive test does not confirm the presence of group A Beta-hemolytic streptococci.

Question 24

Trimethoprim–sulfamethoxazole (SXT) disks are used along with bacitracin disks to differentiate which streptococci?
A. α-Hemolytic streptococci
B. β-Hemolytic streptococci
C. Streptococcus pneumoniae
D. Enterococcus faecalis

Answer 24

B. Beta-Hemolytic streptococci

Note: Beta-Hemolytic streptococci are the only streptococci that should be tested. S. pneumoniae, which is alpha-hemolytic, is susceptible to small concentrations of bacitracin, as are other alpha-hemolytic streptococci. SXT is inhibitory to most streptococci except Streptococcus 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.

Question 25

β-Hemolytic streptococci, not of group A or B, usually exhibit which of the following reactions? S (susceptible) R (resistant)
A. Bacitracin - S, SXT - R
B. Bacitracin - R, SXT - R
C. Bacitracin - R, SXT - S
D. Bacitracin - S or R, SXT - S

Answer 25

D. Bacitracin - S or R, SXT - S

Note: Streptococci that are not group A or B may be either resistant or susceptible to bacitracin but are usually susceptible to SXT.

Question 26

27. A false-positive CAMP test for the presumptive identification of group B streptococci may occur if the plate is incubated in a(n):
    A. Candle jar or CO2 incubator
    B. Ambient air incubator
    C. 35°C incubator
    D. 37°C incubator

Answer 26

A. Candle jar or CO2 incubator

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

Question 27

Which test is used to differentiate the viridans streptococci from the group D streptococci and enterococci?
A. Bacitracin disk test
B. CAMP test
C. Hippurate hydrolysis test
D. Bile esculin test

Answer 27

D. Bile esculin test

Note: 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 is inoculated on the surface and incubated for 24-48 hours in a nonCO2 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).

Question 28

The bile solubility test causes the lysis of:
A. Streptococcus bovis colonies on a blood agar plate
B. Streptococcus pneumoniae colonies on a blood agar plate
C. Group A streptococci in broth culture
D. Group B streptococci in broth culture

Answer 28

B. Streptococcus pneumoniae colonies on a blood agar plate

Note: 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 min, leaving a partially hemolyzed are 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.

Question 29

S. pneumoniae and the viridans streptococci can be
differentiated by which test?
A. Optochin disk test, 5 μg/mL or less
B. Bacitracin disk test, 0.04 unit
C. CAMP test
D. Bile esculin test

Answer 29

A. Optochin disk test, 5 ug/mL or less

Note: Optochin at a concentration of 5 ug/mL or less inhibits the growth of S. pneumoniae but not viridans streptococci. However, Optochin at a concentration in excess of 5 ug/mL inhibits viridans streptococci as well. 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.

Question 30

The salt tolerance test (6.5% salt broth) is used to presumptively identify:
A. Streptococcus pneumoniae
B. Streptococcus bovis
C. Streptococcus equinus
D. Enterococcus faecalis

Answer 30

D. Enterococcus faecalis

Note: Enterococcus 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.

Question 31

In addition to Enterococcus faecalis, which other streptococci with grow in 6.5% salt broth?
A. Group A streptococci
B. Group B streptococci
C. Streptococcus pneumoniae
D. Group D streptococci (nonenterococci)

Answer 31

B. Group B streptococci

Note: 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.

Question 32

The quellung test is used to identify which Streptococcus species?
A. S. pyogenes
B. S. agalactiae
C. S. sanguis
D. S. pneumoniae

Answer 32

D. S. pneumoniae

Note: 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.

Question 33

The L-pyrrolidonyl-β-napthylamide (PYR) hydrolysis test is a presumptive test for which streptococci?
A. Group A and D (enterococcus) streptococci
B. Group A and B β-hemolytic streptococci
C. Nongroup A or B β-hemolytic streptococci
D. Streptococcus pneumoniae and group D streptococci (nonenterococcus)

Answer 33

A. Group A and D (enterococcus) streptococci

Note: The PYR hydrolysis test is highly specific for group A streptococci and group D enterococci. The test detects the pyrrolidonylarylamidase enzyme, which hydrolyzes PYR.

Question 34

A pure culture of β-hemolytic streptococci recovered from a leg wound ulcer gave the following reactions:
CAMP test = Neg
Hippurate hydrolysis = Neg
Bile esculin = Neg 6.5% salt = Neg
PYR = Neg Bacitracin = Resistant
Optochin = Resistant SXT = Sensitive
The most likely identification is:
A. Group A streptococci
B. Group B streptococci
C. Enterococcus faecalis
D. Nongroup A, nongroup B, nongroup D streptococci

Answer 34

D. Nongroup A, nongroup B, nongroup D streptococci

Note: The Beta-hemolytic streptococci - not of groups A, B, or D - 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 Enterococcus faecalis are PYR positive. Enterococcus faecalis is also positive for bile esculin and 6.5% salt broth.

Question 35

β-Hemolytic streptococci, more than 50,000 col/mL, were isolated from a urinary tract catheter urine specimen. Given the following reactions, what is the most likely identification?
CAMP test = Neg
Hippurate hydrolysis = ±
Bile solubility = Neg 6.5% salt = +
PYR = + Bile esculin = +
SXT = Resistant Bacitracin = Resistant
Optochin = Resistant
A. Group A streptococci
B. Group B streptococci
C. Enterococcus faecalis
D. Nongroup A, nongroup B, nongroup D streptococci

Answer 35

C. Enterococcus faecalis

Note: 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 nongroup A, B, or D streptococci will not grow in 6.5% salt broth and are sensitive to SXT. Some group D streptococci will hydrolyze hippurate. Enterococcus faecalis is positive for bile esculin, 6.5% salt broth, and PYR.

Question 36

Nutritionally variant streptococci (NVS) require specific thiol compounds, cysteine, or the active form of vitamin B6. Which of the following tests
supplies these requirements?
A. CAMP test
B. Bacitracin susceptibility test
C. Bile solubility test
D. Staphylococcal cross-streak test

Answer 36

D. Staphylococcal cross-streak test

Note: 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

Question 37

Many α-hemolytic streptococci recovered from a wound were found to be penicillin resistant. Given the following results, what is the most likely identification?
Bile esculin = + PYR = + 6.5% salt = +
Hippurate hydrolysis = +
Bile solubility= Neg SXT = Resistant
A. Enterococcus faecalis
B. Streptococcus pneumoniae
C. Streptococcus bovis
D. Group B streptococci

Answer 37

A. Enterococcus faecalis

Note: 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.

Question 38

Which two tests best differentiate S. bovis (group D, nonenterococcus) from Streptococcus salivarius?
A. Bile esculin and 6.5% salt broth
B. Starch hydrolysis and acid production from mannitol
C. Bacitracin and PYR
D. Trimethoprim-sulfamethoxazole susceptibility and PYR

Answer 38

B. Starch hydrolysis and acid production from mannitol

Note: 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.

Question 39

Two blood cultures on a newborn grew β-hemolytic streptococci with the following reactions:
CAMP test = +
Hippurate hydrolysis = +
Bile solubility = Neg 6.5% salt = +
Bacitracin = Resistant
Bile esculin = Neg PYR = Neg
Trimethoprim–sulfamethoxazole = Resistant
Which is the most likely identification?
A. Group A streptococci
B. Group B streptococci
C. Group D streptococci
D. Nongroup A, nongroup B, nongroup D streptococci

Answer 39

B. Group B streptococci

Note: Group B streptococci (S. agalactiae) are resistant to both bacitracin and SXT. Unlike group A and group D streptococci, the group B streptococci are negative for PYR. With some exceptions, group B streptococci will grow in 6.5% salt broth.

Question 40

MTM medium is used primarily for the selective recovery of which organism from genital specimens?
A. Neisseria gonorrhoeae
B. Neisseria lactamica
C. Neisseria sicca
D. Neisseria flavescens

Answer 40

A. Neisseria gonorrhoeae

Note: Both N. gonorrhoeae and N. meningitidis grow selectively on MTM owing to 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.

Question 41

Variation in colony types seen with fresh isolates of Neisseria gonorrhoeae and sometimes with Neisseria meningitidis are the result of:
A. Multiple nutritional requirements
B. Pili on the cell surface
C. Use of a transparent medium
D. All of these options

Answer 41

D. All of these options

Note: Upon subculture from a primary plate, various sizes and appearances of gonococci are the result of multiple nutritional requirements, such as arginine–hypoxanthine–uracil (AHU)–requiring strains. Colony size and coloration (or light reflection) are the basis of Kellogg’s scheme (types T1 through T5). Types T1 and T2 have pili on the surface and 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.

Question 42

Gram-negative diplococci recovered from an MTM plate and giving a positive oxidase test can be presumptively identified as:
A. Neisseria gonorrhoeae
B. Neisseria meningitidis
C. Neisseria lactamica
D. All of these options

Answer 42

D. All of these options

Note: 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

Question 43

The Superoxol test is used as a rapid presumptive test for:
A. Neisseria gonorrhoeae
B. Neisseria meningitidis
C. Neisseria lactamica
D. Moraxella (Branhamella) catarrhalis

Answer 43

A. Neisseria gonorrhoeae

Note: N. gonorrhoeae colonies recovered from selective MTM media give an immediate 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, Superoxol negative, and has a variable growth pattern on MTM.

Question 44

Nonpathogenic Moraxella spp. capable of growing on selective media for Neisseria can be differentiated from Neisseria spp. by which test?
A. Catalase test
B. 10-unit penicillin disk
C. Oxidase test
D. Superoxol test

Answer 44

B. 10-unit penicillin disk

Note: 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 species form long filaments or long spindle-shaped cells when grown near a 10-unit penicillin disk.

Question 45

A Gram stain of a urethral discharge from a man showing extracellular and intracellular gram-negative diplococci within segmented neutrophils is a presumptive identification for:
A. Neisseria gonorrhoeae
B. Neisseria meningitidis
C. Moraxella (Branhamella) catarrhalis
D. Neisseria lactamica

Answer 45

A. Neisseria gonorrhoeae

Note: A Gram stain of urethral discharge (in men only) showing 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 stain smear.

Question 46

The β-galactosidase test aids in the identification of which Neisseria species?
A. N. lactamica
B. N. meningitidis
C. N. gonorrhoeae
D. N. flavescens

Answer 46

A. N. lactamica

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

Question 47

Cystine tryptic digest (CTA) media used for identification of Neisseria spp. should be inoculated and cultured in:
A. A CO2 incubator at 35°C for 24 hours
B. A CO2 incubator at 42°C for up to 72 hours
C. A nonCO2 incubator at 35°C for up to 72 hours
D. An anaerobic incubator at 35°C for up to 72 hours

Answer 47

C. A nonCO2 incubator at 35°C for up to 72 hours

Note: 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 false-positive (acid) result. The utilization of
carbohydrates by some fastidious gonococcal strains may take up to 72 hours in order to produce a color change in the pH indicator.

Question 48

Culture on MTM media of a vaginal swab produced several colonies of gram-negative diplococci that were catalase and oxidase positive and Superoxol negative. Given the following carbohydrate reactions, select the most likely identification.
Glucose = + Sucrose = Neg Lactose = +
Maltose = + Fructose = Neg
A. Neisseria gonorrhoeae
B. Neisseria sicca
C. Neisseria flavescens
D. Neisseria lactamica

Answer 48

D. Neisseria lactamica

Note: 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.

Question 49

Sputum from a patient with pneumonia produced many colonies of gram-negative diplococci on a
chocolate plate that were also present in fewer numbers on MTM after 48 hours. Given the following results, what is the most likely identification?
Catalase = + Oxidase = +
DNase = + Tributyrin hydrolysis = +
Glucose = Neg Sucrose = Neg
Lactose = Neg Maltose = Neg
Fructose = Neg
A. Moraxella catarrhalis
B. Neisseria flavescens
C. Neisseria sicca
D. Neisseria elongata

Answer 49

A. Moraxella catarrhalis

Note: 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 species.

Question 50

Resistance to which drug categorizes a strain of Staphylococcus aureus as methicillin-resistant Staphylococcus aureus (MRSA)?
A. Oxacillin
B. Colistin
C. Trimethoprim–sulfamethoxazole
D. Tetracycline

Answer 50

A. Oxacillin

Note: 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 staph that are resistant to all of these antibiotic groups. MRSA strains are
treated with vancomycin or oxazolidinone.

Question 51

An oxacillin-disk screen test is used to detect Streptococcus pneumonia resistance to penicillin. Using Mueller–Hinton agar with 5% sheep blood
and a 1 μg oxacillin disk, what is the recommended inhibition zone size for penicillin susceptibility?
A. ≥5 mm
B. ≥10 mm
C. ≥15 mm
D. ≥20 mm

Answer 51

D. greater than or equal to 20 mm

Note: The standard Kirby–Bauer method used for disk diffusion susceptibility testing recommended by CLSI is ≥ 20 mm. The test requires 20–24-hour incubation at 35°C in 5%–7% CO2. If the zone size is ≤ 19 mm, an MIC should be performed on isolates from CSF or blood.

Question 52

Which one of the following organisms is a known producer of β-lactamase–producing strains, and should be tested (screened) by a commercial
β-lactamase assay prior to susceptibility testing?
A. Streptococcus pneumoniae
B. Group B streptococci
C. Enterococcus spp.
D. Planococcus spp.

Answer 52

C. Enterococcus spp.

Note: 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).

Question 53

Which test is used for the determination of inducible clindamycin resistance in staphylococci and streptococci?
A. E-test
B. D zone test
C. A-test
D. CAMP test

Answer 53

B. D zone test

Note: 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. CLSI recommends that inducible clindamycin resistant strains be reported as resistant with a comment that clindamycin may still be effective in some patients.