MLT GlobeBacteriology: Gram-positive cocci – page 5
109 Bacteriology MCQs on Gram-positive cocci with answers and explanations.
CLSI does not accept vancomycin disk diffusion for Staphylococcus aureus because this method:
Zone sizes of VISA isolates overlap with those of susceptible strains, so disk diffusion misses VISA. An MIC method is required for vancomycin testing of S. aureus.
A Streptococcus pneumoniae isolate from sputum gives a 14 mm zone around a 1 µg oxacillin disk. What should the laboratory do next?
An oxacillin zone ≥20 mm predicts penicillin susceptibility. A zone ≤19 mm does not prove resistance, because it can occur with susceptible or resistant strains, so a penicillin MIC must be performed.
The rapid pneumococcal urinary antigen test detects which component of Streptococcus pneumoniae?
The immunochromatographic urine test detects C-polysaccharide (cell wall teichoic acid) shared by all serotypes. It is not serotype specific and does not detect antibody.
Which result shows high-level gentamicin resistance in Enterococcus faecalis, meaning ampicillin plus gentamicin will not be synergistic?
High-level aminoglycoside resistance is screened with 500 µg/mL gentamicin in broth (or a 120 µg disk). Standard 10 µg disks and low concentrations are not used for enterococci because low-level resistance is intrinsic.
An Enterococcus faecalis from urine appears susceptible to cefazolin in a disk diffusion test. How should this be reported?
Enterococci are intrinsically resistant to cephalosporins because of low-affinity penicillin-binding proteins. These drugs may look active in vitro but fail clinically, so CLSI says they must not be reported as susceptible.
Which test result helps separate Enterococcus faecium from Enterococcus faecalis?
E. faecium ferments arabinose, while E. faecalis does not. PYR, 6.5% NaCl growth and bile esculin are positive for both species and are used to identify the genus.
On Granada medium incubated anaerobically, beta-hemolytic Streptococcus agalactiae is recognized by:
Granada medium induces beta-hemolytic group B streptococci to produce an orange-red carotenoid pigment. Rare non-hemolytic strains do not form pigment and may be missed.
Guests vomit about 3 hours after eating cream pastries left at room temperature. Reheating the pastries would not have prevented illness because the cause is:
Staphylococcal enterotoxins are preformed in food and resist heating, so symptoms start within 1–6 hours. Illness from toxins made in the gut after ingestion has a longer incubation period.
An alpha-hemolytic streptococcus gives a 10 mm zone around a 6 mm optochin disk incubated in 5% CO2. What is the next step?
A zone ≥14 mm around a 6 mm disk presumptively identifies S. pneumoniae. Smaller zones are equivocal, because some pneumococci are optochin resistant, so bile solubility is used to confirm.
A group B streptococcus is isolated from a pregnant woman with a severe penicillin allergy (high risk of anaphylaxis). What additional test should the laboratory perform?
For penicillin-allergic women at high anaphylaxis risk, clindamycin may be used for intrapartum prophylaxis, so clindamycin testing with detection of inducible resistance is required. GBS remain penicillin susceptible.
In the CAMP test, a Staphylococcus aureus streak is used because it produces:
The CAMP factor of group B streptococci acts together with staphylococcal beta-lysin to give an arrowhead of enhanced hemolysis. Coagulase and DNase play no part in this reaction.
A 7-year-old with pharyngitis has a negative rapid antigen detection test for group A streptococcus. According to IDSA guidance, what should follow?
Rapid antigen tests are specific but not fully sensitive, so a negative result in children and adolescents should be backed up by culture (or a sensitive molecular test). ASO shows past infection, not current pharyngitis.
Routine susceptibility testing to penicillin is not needed for Streptococcus pyogenes because:
Group A streptococci remain uniformly susceptible to penicillin and other beta-lactams, so testing is not routinely needed. Macrolide or clindamycin testing is still useful for penicillin-allergic patients.
A beta-haemolytic, catalase-negative coccus from a throat swab is PYR positive. Which organism is most likely?
Among beta-haemolytic streptococci, group A S. pyogenes is PYR positive; S. agalactiae is PYR negative and S. aureus is catalase positive.
The slide coagulase test detects which staphylococcal product?
Clumping factor on the cell surface makes cells clump in plasma on a slide; the tube test detects free coagulase.
Which statement about streptococci and related cocci is INCORRECT?
Most viridans streptococci lack a Lancefield antigen or are non-groupable; they are identified by biochemical tests, MALDI-TOF or sequencing.
A coagulase-negative staphylococcus from blood of a patient with native-valve endocarditis is PYR positive and ornithine decarboxylase positive. The most likely species is:
S. lugdunensis is PYR and ornithine decarboxylase positive and behaves aggressively like S. aureus in endocarditis. S. epidermidis is PYR negative in most strains and less virulent.
An Enterococcus faecium from a rectal screening swab is highly resistant to both vancomycin and teicoplanin. Which gene is most likely responsible?
vanA gives high-level resistance to both vancomycin and teicoplanin and is the commonest VRE gene. vanB strains usually remain teicoplanin susceptible.
A motile enterococcus identified as E. gallinarum has a vancomycin MIC of 8 µg/mL and is teicoplanin susceptible. This resistance is best explained by:
E. gallinarum and E. casseliflavus carry intrinsic vanC, giving low-level vancomycin resistance. They are motile, unlike E. faecalis and E. faecium, and have less infection-control significance.
A catalase-negative gram-positive coccus in pairs and short chains is intrinsically vancomycin resistant and produces gas from glucose in MRS broth. It is most likely:
Leuconostoc is intrinsically vancomycin resistant and produces gas from glucose. Pediococcus is also vancomycin resistant but forms tetrads and produces no gas.