MLT GlobeBacteriology: Gram-positive cocci – page 6
109 Bacteriology MCQs on Gram-positive cocci with answers and explanations.
Pus from a liver abscess grows tiny beta-hemolytic streptococcal colonies with a caramel (butterscotch) odour. The organism most likely belongs to the:
The S. anginosus group forms minute colonies with a caramel odour and is known for causing deep abscesses. Other viridans groups rarely produce this picture.
Blood cultures from a patient with endocarditis show gram-positive cocci in chains. On subculture, colonies grow on sheep blood agar only as tiny satellites next to a Staphylococcus aureus streak. The most likely organism is:
Nutritionally variant streptococci (Abiotrophia, Granulicatella) need pyridoxal or L-cysteine, which S. aureus supplies, so they satellite. S. mitis and enterococci grow well on sheep blood agar alone.
A dog bite wound grows a catalase-positive coccus in clusters. It is tube coagulase positive, PYR positive and Voges-Proskauer negative. The most likely organism is:
S. pseudintermedius, a canine commensal, is coagulase positive but PYR positive and VP negative. S. aureus is usually PYR negative and VP positive; S. lugdunensis and S. epidermidis are tube coagulase negative.
Urine from an 80-year-old man grows alpha-hemolytic, catalase-negative gram-positive cocci in clusters and tetrads. The isolate is PYR negative, LAP positive and grows in 6.5% NaCl. The most likely organism is:
Aerococcus urinae is a catalase-negative coccus in clusters that is PYR negative and LAP positive; A. viridans has the reverse pattern. Enterococci are PYR positive and grow in chains; staphylococci are catalase positive.
Sputum from a patient with cystic fibrosis grows tiny, non-pigmented, non-hemolytic colonies after 48–72 hours. They are catalase-positive cocci in clusters and weakly tube coagulase positive. The most likely explanation is:
S. aureus small-colony variants grow slowly, lose pigment and hemolysis, and often show weak or delayed coagulase. They are often thymidine- or hemin-dependent and persist in cystic fibrosis airways.
A Staphylococcus aureus isolate is resistant to cefoxitin by disk diffusion, but mecA PCR and a PBP2a latex test are both negative. The most likely explanation is:
mecC encodes a divergent PBP2a that causes cefoxitin resistance but is missed by mecA-specific PCR and many PBP2a latex tests. Penicillinase alone does not cause cefoxitin resistance.
A Staphylococcus aureus penicillin disk test gives a 30 mm zone (susceptible range), but the zone edge is sharp and cliff-like. What should be reported?
CLSI penicillin zone-edge test: a sharp, cliff-like edge means beta-lactamase production, so the isolate is reported penicillin resistant even with a large zone. A fuzzy, beach-like edge means beta-lactamase negative.
A catalase-negative gram-positive coccus in pairs and tetrads is vancomycin resistant, PYR negative and LAP positive. It does not produce gas in MRS broth. The most likely genus is:
Pediococcus is intrinsically vancomycin resistant, LAP positive, PYR negative and makes no gas. Leuconostoc is also vancomycin resistant but produces gas and is LAP negative; aerococci are vancomycin susceptible.
Daptomycin results should not be reported for MRSA isolated from sputum because daptomycin:
Pulmonary surfactant binds and inactivates daptomycin, so it fails in pneumonia even when the isolate tests susceptible. It remains useful for bacteremia and skin infections caused by MRSA.