Streptococcus Questions and Answers

Answer: C

Correct answer: Streptococcus mutans

S. mutans is an oral organism and is not a urinary pathogen. The others are all found in urine cultures: E. coli overwhelmingly the commonest, S. aureus in catheterised or bacteraemic patients, and S. agalactiae particularly in pregnancy, where its presence in urine indicates heavy genital colonisation.

Answer: B

Correct answer: Streptococcus agalactiae

Group B streptococcus is carried in the maternal genital tract and is the leading cause of early onset neonatal sepsis and pneumonia, acquired as the baby passes through the birth canal. Prematurity is a recognised risk factor and maternal urinary tract infection points to heavy colonisation. The beta-haemolytic colonies are consistent, and screening in late pregnancy exists to prevent exactly this.

Answer: C

Correct answer: S. saprophyticus is normal flora of humans skin surface

S. saprophyticus belongs to the genitourinary flora rather than being a general skin commensal; it is S. epidermidis that colonises skin everywhere. The distinction matters clinically, because S. saprophyticus in a urine culture from a young woman is a genuine pathogen while S. epidermidis in a blood culture is more often a contaminant.

Answer: A

Correct answer: polysaccharide capsule

The pneumococcal capsule prevents phagocytosis and is the organism’s principal virulence factor: unencapsulated strains are avirulent. It is also the basis of prevention, since both the polysaccharide and the conjugate vaccines are directed against it, and of typing, there being more than ninety capsular serotypes. M protein and the streptolysins belong to S. pyogenes.

Answer: C

Correct answer: Streptococcus pyogens (A)

S. pyogenes is regarded as a pathogen rather than a commensal: asymptomatic throat carriage occurs, particularly in children, but it is not part of the established normal flora and its presence is significant. The other three are genuine commensals - S. aureus in the anterior nares, S. agalactiae in the genital and gastrointestinal tracts, E. faecalis in the bowel.

Answer: A

Correct answer: A

S. pyogenes carries the group A cell wall carbohydrate, which is what the Lancefield grouping detects and what gives it its everyday name of group A streptococcus. Group B is S. agalactiae, the neonatal pathogen, and group D covers the enterococci and S. bovis.

Answer: A

Correct answer: Enterococcus faecium

Vancomycin resistance is far commoner in E. faecium than in E. faecalis, and vancomycin-resistant E. faecium is one of the principal hospital-acquired resistance problems. The resistance comes from van gene clusters that replace the terminal D-alanine of the cell wall precursor, so vancomycin can no longer bind.

Answer: A

Correct answer: Streptococcus pneumoniae

S. pneumoniae is the commonest bacterial cause of acute otitis media, as it is of sinusitis and community-acquired pneumonia - all infections spreading from nasopharyngeal carriage into an adjacent space. Conjugate vaccination has reduced the incidence of all three, which is indirect confirmation of the organism’s role.

Answer: B

Correct answer: False

The staphylococcal abscess is designed to keep them out. Coagulase converts fibrinogen to fibrin around the lesion, producing an avascular wall that antibiotics cannot cross in useful concentration, and the organisms inside are metabolically slow and correspondingly tolerant. This is why an abscess must be drained surgically and why antibiotic therapy alone so often fails.

Answer: D

Correct answer: Found only in the oral cavities as commensals

Viridans streptococci are commensals of the mouth but also of the pharynx, the gastrointestinal tract and the female genital tract, so “only in the oral cavities” is too narrow. The other three statements hold: most are alpha-haemolytic, they are the principal agents of dental caries, and they carry no Lancefield group antigen.