MLT GlobeBacteriology: Gram-positive cocci
109 Bacteriology MCQs on Gram-positive cocci with answers and explanations.
Gram-positive cocci from a pus sample bubble vigorously when mixed with hydrogen peroxide. This result separates:
Staphylococci produce catalase, releasing oxygen bubbles from H2O2; streptococci and enterococci are catalase-negative. Neisseria and Moraxella are both catalase-positive.
Panton-Valentine toxin of S. aureus forms pores that kill neutrophils and macrophages. This class of exotoxin is called:
Leucocidins are exotoxins that destroy white blood cells. Enterotoxins act on the gut, neurotoxins on nerves; antitoxins are antibodies that neutralise toxins.
An alpha-haemolytic Gram-positive diplococcus shows a zone of inhibition around an ethylhydrocupreine disc. This identifies Streptococcus pneumoniae, which is sensitive to:
Optochin (ethylhydrocupreine) susceptibility separates S. pneumoniae from other viridans streptococci. Bacitracin screens S. pyogenes; novobiocin separates S. saprophyticus.
A beta-haemolytic, catalase-negative Gram-positive coccus from a vaginal swab gives an arrowhead of enhanced haemolysis near a S. aureus streak. It is:
The CAMP factor of group B streptococci synergises with staphylococcal beta-lysin, giving the arrowhead pattern, the basis of the CAMP test.
For the routine slide catalase test on staphylococci, which hydrogen peroxide concentration is used?
3% H2O2 is standard for catalase testing. 30% (superoxol) is used for Neisseria gonorrhoeae identification.
Several neonates in a nursery develop S. aureus sepsis. Screening staff for carriage should focus on which body site?
The anterior nose is the main reservoir of S. aureus in healthy carriers, and nasal carriers often spread it via hands.
Which organism is NOT a recognised cause of infectious diarrhoea?
E. faecalis is a gut commensal causing UTI and endocarditis, not diarrhoea. C. perfringens enterotoxin, diarrhoeagenic E. coli and V. cholerae all cause diarrhoea.
A plasma tube test is used on Gram-positive cocci from a blood culture. It separates which organisms?
Free coagulase clots rabbit plasma; S. aureus is positive, while S. epidermidis and other CoNS are negative.
A catalase- and coagulase-positive coccus from a boil forms golden-yellow colonies on nutrient agar. The organism is:
S. aureus produces the carotenoid staphyloxanthin, giving golden colonies. S. pyogenes forms small grey colonies; E. coli and C. diphtheriae are not golden.
Which statement about Streptococcus pneumoniae (formerly Diplococcus pneumoniae) is INCORRECT?
Pneumococci are lancet-shaped Gram-positive cocci in pairs or short chains; the polysaccharide capsule is the main virulence factor.
A throat swab grows catalase-negative gram-positive cocci in chains with wide zones of complete haemolysis on blood agar; the isolate reacts with Lancefield group A antiserum. Identify it.
Group A beta-haemolytic streptococcus is S. pyogenes. S. agalactiae is group B, S. pneumoniae is alpha-haemolytic, and E. faecalis carries group D antigen and is usually non-haemolytic.
Gram-positive cocci in grape-like clusters that are catalase-positive belong to which genus, rather than Streptococcus?
Staphylococci are catalase-positive and form clusters. Streptococci, enterococci and lactococci are catalase-negative and grow in chains or pairs.
An S. aureus isolate carries mecA, encoding PBP2a, and is resistant to oxacillin and most beta-lactams. It is reported as:
Methicillin-resistant S. aureus has altered penicillin-binding protein 2a with low beta-lactam affinity. VISA refers to reduced vancomycin susceptibility, not mecA.
More than 100 serotypes of Streptococcus pneumoniae are recognised. The typing is based on:
Pneumococcal serotypes differ in capsular polysaccharide, detected by the Quellung reaction and targeted by vaccines. Pneumococci are non-motile.
Gram-positive cocci dividing in three planes form cubical packets of eight cells. This arrangement is called:
Division in three perpendicular planes gives packets of eight (sarcinae). Two planes give tetrads; irregular planes give clusters.
Clearing around colonies on DNA–methyl green agar is used to support identification of:
S. aureus produces heat-stable DNase (thermonuclease). Coagulase-negative staphylococci are usually DNase-negative.
Which disc test presumptively separates group A from other beta-haemolytic streptococci?
Group A (S. pyogenes) is bacitracin-susceptible, while most other beta-haemolytic groups are resistant. Optochin identifies pneumococci; novobiocin is for S. saprophyticus.
Several people vomit 2–4 hours after eating custard pastries. The most likely cause is:
Preformed heat-stable staphylococcal enterotoxin causes vomiting within 1–6 hours. Infections needing multiplication have incubations of a day or more.
Which group of infections is most characteristic of Staphylococcus aureus?
S. aureus causes localised pyogenic infections (furuncles, abscesses, bullous impetigo). Scarlet fever is caused by S. pyogenes.
Which streptococcus is typically beta-haemolytic on sheep blood agar?
S. pyogenes produces streptolysins giving complete haemolysis. S. salivarius and S. mitis are usually non- or alpha-haemolytic; S. pneumoniae is alpha-haemolytic.