MLT GlobeBacteriology: Gram-positive cocci – page 4
109 Bacteriology MCQs on Gram-positive cocci with answers and explanations.
Which statement about Streptococcus pneumoniae is correct?
The polysaccharide capsule is its main virulence factor. Pneumococci autolyse and may appear gram-variable in old cultures, and are non-motile.
The Quellung (capsular swelling) reaction would be negative with which organism?
Quellung detects capsules via antiserum-induced swelling. S. Typhimurium lacks a capsule; the others are encapsulated.
MRSA resists nearly all beta-lactams because mecA encodes:
PBP2a has low affinity for beta-lactams, so wall synthesis continues. Efflux and gyrase mutations explain tetracycline and quinolone resistance.
Streptococcus suis, a cause of meningitis and deafness in humans, is acquired mainly from:
S. suis is a swine pathogen; pig farmers and people who handle or eat raw pork are at risk. Person-to-person spread is not recognised.
A beta-hemolytic streptococcus from a vaginal-rectal swab of a pregnant woman is CAMP positive and hippurate positive. It belongs to:
Group B streptococci give a positive CAMP test (arrowhead hemolysis with S. aureus) and hydrolyse hippurate. They are screened in pregnancy because they cause neonatal sepsis and meningitis.
The tube coagulase test for Staphylococcus aureus detects which product?
The tube test detects free (extracellular) coagulase, which activates prothrombin to form a clot in plasma. Bound coagulase (clumping factor) is detected by the slide test.
According to CLSI, which disk is used in disk diffusion to predict mecA-mediated oxacillin resistance in Staphylococcus aureus?
Cefoxitin is a strong inducer of mecA and gives clearer, more reproducible results than oxacillin disks, so CLSI recommends the cefoxitin disk as the surrogate test for MRSA.
An S. aureus isolate is erythromycin resistant and clindamycin susceptible. In a D-zone test, the clindamycin zone is flattened on the side facing the erythromycin disk. What should be reported?
A D-shaped zone shows inducible MLSB resistance (erm gene). Clindamycin may fail during therapy, so it is reported as resistant.
A 9-year-old develops glomerulonephritis 3 weeks after impetigo. Which antibody test best supports recent group A streptococcal skin infection?
After skin infection the ASO response is often weak because skin lipids inactivate streptolysin O. Anti-DNase B rises reliably after both pharyngeal and skin infections.
When plating a throat swab on sheep blood agar, the agar is stabbed with the loop. The main reason is to:
Streptolysin O is inactivated by oxygen, so hemolysis is strongest below the agar surface. Stabbing reveals subsurface beta-hemolysis of group A streptococci.
Which test best separates Enterococcus spp. from Streptococcus gallolyticus (group D non-enterococcus)?
Both groups hydrolyze bile esculin, but only enterococci grow in 6.5% NaCl. Both are catalase negative and optochin resistant.
Colonies of Streptococcus pneumoniae often develop a sunken (umbilicated) centre after 24–48 hours. This is caused by:
The autolysin (LytA) lyses older cells in the colony centre, producing the 'draughtsman' appearance. The same enzyme is responsible for bile solubility.
For group B streptococcus screening, CDC-recommended processing of a vaginal-rectal swab is:
Selective enrichment broth (e.g., Lim broth) increases detection of low numbers of GBS. Direct plating alone misses many carriers.
A catalase-positive gram-positive coccus in tetrads forms bright yellow colonies, is modified oxidase positive and susceptible to 0.04 U bacitracin. It is most likely:
Micrococcus is modified oxidase (microdase) positive and bacitracin susceptible; staphylococci are negative and resistant. Aerococcus is catalase negative.
A newborn has widespread blistering and peeling of skin with a positive Nikolsky sign; blister fluid is sterile. The toxin involved acts on:
Staphylococcal exfoliative toxins cleave desmoglein-1, causing scalded skin syndrome. The toxin is produced at a distant site, so blister fluid is usually sterile.
A large-colony beta-hemolytic streptococcus must be separated from S. dysgalactiae (group C/G). Which single test is most specific for S. pyogenes among these?
Among beta-hemolytic streptococci, PYR positivity is specific for S. pyogenes. Some group C and G strains are bacitracin susceptible, so bacitracin is less specific.
Blood cultures from a 68-year-old grow Streptococcus gallolyticus (formerly S. bovis biotype I). The laboratory result should prompt investigation for:
S. gallolyticus bacteremia or endocarditis is strongly associated with colonic tumours, so colonoscopy is recommended.
Rapid latex agglutination kits for identifying Staphylococcus aureus usually detect:
Latex particles coated with fibrinogen and IgG bind clumping factor and protein A on the S. aureus surface. Free coagulase is detected by the tube test.
A beta-hemolytic, catalase-negative coccus from a throat swab is susceptible to a 0.04 U bacitracin disk and resistant to trimethoprim-sulfamethoxazole (SXT). The presumptive identification is:
Group A streptococci are bacitracin susceptible and SXT resistant. Group B streptococci are resistant to both, while groups C and G are usually bacitracin resistant and SXT susceptible.
A broth microdilution test gives a vancomycin MIC of 4 µg/mL for a Staphylococcus aureus blood isolate. Using CLSI breakpoints, this result is interpreted as:
CLSI vancomycin breakpoints for S. aureus are ≤2 µg/mL susceptible, 4–8 µg/mL intermediate and ≥16 µg/mL resistant. An MIC of 4 µg/mL is therefore VISA, not VRSA.