MLT GlobeBacteriology: Gram-positive cocci – page 3
109 Bacteriology MCQs on Gram-positive cocci with answers and explanations.
Colonies of Streptococcus pneumoniae dissolve when a few drops of which reagent are added?
Bile salts activate the pneumococcal autolysin, so pneumococci are bile soluble, unlike other alpha-haemolytic streptococci.
Gram-positive cocci arranged in chains most likely belong to which genus?
Streptococci divide in one plane and form chains; staphylococci form clusters and micrococci form tetrads.
Which organism is the most common cause of acute osteomyelitis?
S. aureus causes most cases of acute osteomyelitis in all age groups.
Which bacterium is the most common cause of community-acquired lobar pneumonia?
The pneumococcus is the classic and most frequent cause of community-acquired lobar pneumonia.
On a Gram-stained sputum smear, Streptococcus pneumoniae typically appears as:
Pneumococci are elongated, pointed (lancet-shaped) cocci in pairs, often with a capsule halo.
Necrotising fasciitis ('flesh-eating' infection) is classically caused by:
Group A streptococcus is the classic cause of rapidly spreading necrotising fasciitis.
Which organism is a leading cause of sepsis and meningitis in newborns during the first week of life?
Group B streptococci colonise the maternal genital tract and are a major cause of early-onset neonatal sepsis and meningitis.
A raised antistreptolysin O (ASO) titre supports recent infection with:
Streptolysin O is produced by group A streptococci, so a raised ASO titre supports recent S. pyogenes infection, for example before rheumatic fever.
Enterococci are part of the normal flora mainly of the:
Enterococci are normal gut bacteria, which is why they cause urinary, abdominal and wound infections.
A suprapubic urine is plated with a calibrated 0.01 mL (10 µL) loop and yields 10 colonies of Staphylococcus saprophyticus. What is the colony count?
CFU/mL = colonies x 1/loop volume = 10 x 100 = 1,000 CFU/mL. Any growth from suprapubic aspirate is significant.
Enterococcus faecalis endocarditis is treated with a combination that is bactericidal through synergy. Which is the classic synergistic pair?
The cell-wall-active penicillin increases aminoglycoside uptake, giving bactericidal synergy against enterococci unless high-level aminoglycoside resistance is present.
Alpha-haemolytic colonies on blood agar become flattened with a sunken centre after 48 h, owing to autolysis. Which organism produces these 'draughtsman' colonies?
Pneumococcal autolysin causes central collapse, giving draughtsman or crater colonies. Enterococci and Moraxella form convex colonies.
Which property is characteristic of Enterococcus faecalis?
Enterococci tolerate 6.5% salt and are bile-aesculin positive. Optochin susceptibility and bile solubility identify S. pneumoniae; enterococci are catalase-negative.
Which toxin acts as a superantigen, linking MHC class II to T-cell receptors non-specifically?
TSST-1 of S. aureus is a superantigen causing massive cytokine release. Exotoxin A and diphtheria toxin inhibit EF-2; tetanospasmin blocks inhibitory neurotransmitter release.
A patient with Staphylococcus aureus bacteraemia is suspected of endocarditis or deep metastatic abscess. The key laboratory investigation is:
Persistent positive blood cultures indicate an endovascular or metastatic focus. ASO is for streptococci and Widal for typhoid; S. aureus has no endotoxin.
Most clinical isolates of which organism are resistant to benzylpenicillin because they produce penicillinase?
Over 80–90% of S. aureus strains produce beta-lactamase. S. pyogenes and T. pallidum remain penicillin-susceptible.
An infection-control team investigates a suspected MRSA ward outbreak. Current strain-relatedness is best determined by:
WGS (or PFGE/spa typing) provides high-resolution discrimination between strains. Phage typing is obsolete and antibiograms are too crude.
Which test best separates Micrococcus from coagulase-negative staphylococci?
Micrococcus contains cytochrome c and is microdase-positive; staphylococci are negative. Both genera are catalase-positive gram-positive cocci.
Gram-positive cocci that are larger than staphylococci and typically arranged in tetrads are:
Micrococci are about 1–1.8 µm and often form tetrads, and are commonly skin contaminants. Staphylococci form clusters; streptococci chains.
Lancefield serological grouping is applied mainly to streptococci that are:
Lancefield grouping detects cell-wall carbohydrate antigens of beta-haemolytic streptococci (A, B, C, G). Pneumococci are alpha-haemolytic and not grouped this way.