Three of three blood culture sets from a 67-year-old man with a prosthetic aortic valve grow the same catalase-positive, gram-positive coccus in clusters. Results: tube coagulase negative at 4 and 24 hours; slide test for clumping factor negative; PYR negative; ornithine decarboxylase negative; novobiocin susceptible. The broth microdilution oxacillin MIC is 1 µg/mL. CLSI M100 oxacillin MIC breakpoints (µg/mL) are: Staphylococcus aureus and S. lugdunensis, susceptible 2 or less and resistant 4 or more; S. epidermidis, susceptible 0.25 or less and resistant 0.5 or more. How should oxacillin be reported?

Resistant, by the S. epidermidis breakpoint

Intermediate, as it falls between the two

Susceptible, by the S. aureus breakpoint

Not reported, as a likely skin contaminant

Why is rabbit plasma with EDTA preferred over citrated plasma for the tube coagulase test?

Organisms that use citrate can clot citrated plasma

EDTA activates prothrombin and makes the clot form faster

EDTA plasma does not need to be refrigerated

Citrate destroys staphylococcal coagulase

Staphylococci can be separated from Micrococcus because staphylococci:

Ferment glucose under anaerobic conditions

Grow on sheep blood agar at 35 °C

Staphylococci and micrococci

15 DOH Abu Dhabi (HAAD) lab licensing practice questions on Staphylococci and micrococci, part of Microbiology. The first 15 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 74-year-old man with no urinary catheter and no recent urological procedure has fever of 38.9 °C and new low back pain. A clean-catch urine grows more than 10^8 CFU/L (10^5 CFU/mL) of a pure, beta-hemolytic, catalase-positive coccus in clusters that clots rabbit plasma within 4 hours. The 30-µg cefoxitin disk zone is 18 mm (CLSI for S. aureus: 22 mm or more susceptible, 21 mm or less resistant). No blood cultures have been collected. How should the isolate be reported, and what should the laboratory advise?
1. AMRSA; advise blood cultures
2. BMSSA; ask for a repeat urine
3. CMRSA; ask for a repeat urine
4. DMSSA; advise blood cultures
Show answerHide answer
Answer: A · MRSA; advise blood cultures
The coagulase-positive staphylococcus is S. aureus, and a cefoxitin zone of 18 mm is below the 21 mm resistant breakpoint, so the isolate is MRSA. S. aureus is an uncommon urinary pathogen in a patient without a catheter or instrumentation, and in that setting bacteriuria often reflects seeding from the bloodstream. With fever and new back pain, bacteremia with a spinal focus must be excluded, so blood cultures should be advised at once. A repeat urine would only delay this.
- B: Both parts are wrong: the zone shows MRSA, and blood cultures are needed.
- C: Pure S. aureus without a catheter suggests bacteremia; a repeat urine only delays blood cultures.
- D: A cefoxitin zone of 18 mm is resistant, so the isolate is MRSA, not MSSA.
Question 2 · Hard
Three of three blood culture sets from a 67-year-old man with a prosthetic aortic valve grow the same catalase-positive, gram-positive coccus in clusters. Results: tube coagulase negative at 4 and 24 hours; slide test for clumping factor negative; PYR negative; ornithine decarboxylase negative; novobiocin susceptible. The broth microdilution oxacillin MIC is 1 µg/mL. CLSI M100 oxacillin MIC breakpoints (µg/mL) are: Staphylococcus aureus and S. lugdunensis, susceptible 2 or less and resistant 4 or more; S. epidermidis, susceptible 0.25 or less and resistant 0.5 or more. How should oxacillin be reported?
1. AIntermediate, as it falls between the two
2. BResistant, by the S. epidermidis breakpoint
3. CSusceptible, by the S. aureus breakpoint
4. DNot reported, as a likely skin contaminant
Show answerHide answer
Answer: B · Resistant, by the S. epidermidis breakpoint
The isolate is a coagulase-negative staphylococcus, and the negative PYR and ornithine results with novobiocin susceptibility point to S. epidermidis rather than S. lugdunensis, which is PYR and ornithine positive. CLSI gives S. epidermidis a much lower oxacillin breakpoint because many mecA-positive strains have MICs of only 0.5 to 2 µg/mL, which the S. aureus breakpoint would call susceptible. An MIC of 1 µg/mL is therefore resistant, and the isolate should also be reported resistant to other standard beta-lactams such as cefazolin. Three of three sets in a patient with a prosthetic valve make it a likely pathogen, not a contaminant.
- A: CLSI has no intermediate category for oxacillin in staphylococci.
- C: The S. aureus breakpoint misses mecA-positive S. epidermidis with low MICs.
- D: Three of three sets in a patient with a prosthetic valve point to true infection.
Question 3 · Medium
Why is rabbit plasma with EDTA preferred over citrated plasma for the tube coagulase test?
1. AEDTA activates prothrombin and makes the clot form faster
2. BEDTA plasma does not need to be refrigerated
3. CCitrate destroys staphylococcal coagulase
4. DOrganisms that use citrate can clot citrated plasma
Show answerHide answer
Answer: D · Organisms that use citrate can clot citrated plasma
Some organisms, such as enterococci and Pseudomonas, can use citrate, which frees the bound calcium so that the plasma clots without any coagulase. In a mixed or misidentified culture, this gives a false-positive tube coagulase. EDTA also binds calcium but is not metabolized, so only true coagulase activity causes a clot. Using a pure, fresh culture and running positive and negative controls also prevents errors.
- A: EDTA binds calcium; it does not activate prothrombin.
- B: Both plasmas need correct storage; this is not the reason.
- C: Citrate does not inactivate coagulase.
Question 4 · Easy
Staphylococci can be separated from Micrococcus because staphylococci:
1. AGrow on sheep blood agar at 35 °C
2. BFerment glucose under anaerobic conditions
3. CGive a positive catalase test
4. DForm yellow-pigmented colonies
Show answerHide answer
Answer: B · Ferment glucose under anaerobic conditions
Staphylococci are facultative anaerobes and produce acid from glucose in a sealed (anaerobic) oxidation-fermentation tube, while Micrococcus is strictly aerobic and does not. Both genera are catalase-positive gram-positive cocci, both can be pigmented and both grow on routine blood agar. Other separating tests are the modified oxidase (microdase), positive in Micrococcus, and the 0.04-U bacitracin disk, which inhibits Micrococcus but not staphylococci. The difference matters because Micrococcus in a blood culture is usually a skin contaminant.
- A: Both genera grow well on sheep blood agar.
- C: Both genera are catalase positive.
- D: Both can form yellow colonies; S. aureus is often golden.
Question 5 · Easy
A urine isolate of catalase-positive, coagulase-negative cocci in clusters gives a 10-mm zone around a 5-µg novobiocin disk (resistant: 16 mm or less). This points to:
1. AStaphylococcus epidermidis
2. BStaphylococcus haemolyticus
3. CStaphylococcus saprophyticus
4. DStaphylococcus hominis
Show answerHide answer
Answer: C · Staphylococcus saprophyticus
Staphylococcus saprophyticus is intrinsically resistant to novobiocin, while S. epidermidis and most other clinically important coagulase-negative staphylococci are susceptible. It causes cystitis in young, sexually active women, so laboratories identify it to species from urine. It is also urease positive. MALDI-TOF identifies it directly, but the novobiocin disk is a cheap and reliable bench screen.
- A: S. epidermidis is novobiocin susceptible.
- B: S. haemolyticus is novobiocin susceptible.
- D: S. hominis is novobiocin susceptible.
Question 6 · Hard
A Staphylococcus aureus blood isolate gives a cefoxitin disk zone of 23 mm (CLSI: 21 mm or less predicts mecA-mediated resistance). A PBP2a latex test on growth from the zone edge is positive. The culture is pure, and both results repeat. How should oxacillin be reported?
1. AIntermediate, because the results disagree
2. BSusceptible, because the disk result decides
3. CNot reported until mecA PCR is done
4. DResistant, because PBP2a was detected
Answer and explanation in LearnFree account
Question 7 · Hard
Catalase-positive cocci in clusters from a 30-year-old woman's urine give these results: slide coagulase, clumping in plasma and also in the saline control drop; tube coagulase, no clot at 4 or 24 hours; colonies white and nonhemolytic. What is the correct conclusion?
1. AStaphylococcus aureus; the slide test is positive
2. BUndetermined; repeat the slide test with more growth
3. CCoagulase negative; the slide test was invalid
4. DStaphylococcus aureus; the tube test failed
Answer and explanation in LearnFree account
Question 8 · Medium
A nasal swab on mannitol salt agar grows yellow colonies surrounded by yellow medium. The gram-positive cocci in clusters are catalase positive. Which test is needed before reporting Staphylococcus aureus?
1. AA bacitracin 0.04-unit disk test
2. BA 5-µg novobiocin disk test
3. CA modified oxidase (microdase) test
4. DA coagulase or S. aureus latex test
Answer and explanation in LearnFree account
Question 9 · Medium
A 3-week-old infant has widespread peeling of the skin that leaves moist red areas, and the mucous membranes are spared. The blisters are culture negative, but Staphylococcus aureus grows from the umbilicus. Which toxin causes this illness?
1. AExfoliative toxins A and B
2. BToxic shock syndrome toxin-1
3. CStaphylococcal enterotoxin B
4. DPanton-Valentine leukocidin
Answer and explanation in LearnFree account
Question 10 · Medium
CLSI recommends a cefoxitin disk, rather than an oxacillin disk, to detect mecA-mediated resistance in Staphylococcus aureus. The main reason is that cefoxitin:
1. AInduces mecA expression and gives clearer zones
2. BGives reliable results at any incubation temperature
3. CIs the drug of choice for treating MRSA infections
4. DAlso detects vancomycin-intermediate strains
Answer and explanation in LearnFree account
Question 11 · Hard
A single blood culture set from a 64-year-old grows a coagulase-negative staphylococcus that MALDI-TOF identifies as Staphylococcus lugdunensis. The best laboratory approach is to:
1. AReport it as a probable skin contaminant without further testing
2. BRegard it as a likely pathogen and apply S. aureus oxacillin breakpoints
3. CRegard it as a likely pathogen and apply S. epidermidis oxacillin breakpoints
4. DDelay any report until a novobiocin disk confirms the species
Answer and explanation in LearnFree account
Question 12 · Medium
A Staphylococcus aureus isolate is PBP2a positive. By CLSI, which beta-lactam may still be reported from its own test result?
1. AMeropenem
2. BCeftaroline
3. CCefazolin
4. DAmoxicillin-clavulanate
Answer and explanation in LearnFree account
Question 13 · Hard
In an external quality assessment, known MRSA strains test cefoxitin susceptible. The procedure was: direct colony suspension to 0.5 McFarland, Mueller-Hinton agar, 30-µg cefoxitin disk, ambient air at 37 °C, read at 18 hours. Which step most likely caused the error?
1. AReading at 18 hours
2. BIncubation at 37 °C
3. CThe 0.5 McFarland inoculum
4. DIncubation in ambient air
Answer and explanation in LearnFree account
Question 14 · Hard
Catalase-positive cocci in clusters from a 50-year-old's surgical wound are negative with a latex kit that detects only clumping factor and protein A; the kit controls react correctly. The tube coagulase with EDTA rabbit plasma clots at 4 hours, and the isolate is mecA positive. The most likely explanation is:
1. AThe isolate is Staphylococcus lugdunensis
2. BSome MRSA strains mask or lack the latex targets
3. CCitrate use by the isolate clotted the plasma
4. DThe isolate is a coagulase-negative species
Answer and explanation in LearnFree account
Question 15 · Easy
The tube coagulase test detects which staphylococcal product?
1. AProtein A on the cell surface
2. BDeoxyribonuclease (DNase)
3. CClumping factor bound to the cell wall
4. DFree coagulase released into the plasma
Answer and explanation in LearnFree account

Staphylococci and micrococci

Why is rabbit plasma with EDTA preferred over citrated plasma for the tube coagulase test?

Staphylococci can be separated from Micrococcus because staphylococci:

A urine isolate of catalase-positive, coagulase-negative cocci in clusters gives a 10-mm zone around a 5-µg novobiocin disk (resistant: 16 mm or less). This points to:

A Staphylococcus aureus blood isolate gives a cefoxitin disk zone of 23 mm (CLSI: 21 mm or less predicts mecA-mediated resistance). A PBP2a latex test on growth from the zone edge is positive. The culture is pure, and both results repeat. How should oxacillin be reported?

Catalase-positive cocci in clusters from a 30-year-old woman's urine give these results: slide coagulase, clumping in plasma and also in the saline control drop; tube coagulase, no clot at 4 or 24 hours; colonies white and nonhemolytic. What is the correct conclusion?

A nasal swab on mannitol salt agar grows yellow colonies surrounded by yellow medium. The gram-positive cocci in clusters are catalase positive. Which test is needed before reporting Staphylococcus aureus?

A 3-week-old infant has widespread peeling of the skin that leaves moist red areas, and the mucous membranes are spared. The blisters are culture negative, but Staphylococcus aureus grows from the umbilicus. Which toxin causes this illness?

CLSI recommends a cefoxitin disk, rather than an oxacillin disk, to detect mecA-mediated resistance in Staphylococcus aureus. The main reason is that cefoxitin:

A single blood culture set from a 64-year-old grows a coagulase-negative staphylococcus that MALDI-TOF identifies as Staphylococcus lugdunensis. The best laboratory approach is to:

A Staphylococcus aureus isolate is PBP2a positive. By CLSI, which beta-lactam may still be reported from its own test result?

In an external quality assessment, known MRSA strains test cefoxitin susceptible. The procedure was: direct colony suspension to 0.5 McFarland, Mueller-Hinton agar, 30-µg cefoxitin disk, ambient air at 37 °C, read at 18 hours. Which step most likely caused the error?

The tube coagulase test detects which staphylococcal product?