Streptococcus Questions and Answers

Answer: A

Correct answer: Streptococcus MG

Patients with primary atypical pneumonia, caused by Mycoplasma pneumoniae, develop agglutinins to Streptococcus MG as well as cold agglutinins to human group O red cells. The reaction is a heterophile cross-reaction and not evidence of streptococcal infection; it was used as a serological aid before specific mycoplasma tests were available.

Answer: A

Correct answer: Bacitracin sensitivity

A bacitracin disc is the classic presumptive test: S. pyogenes is sensitive to low-concentration bacitracin while most other beta-haemolytic streptococci are resistant, so a zone of inhibition around the disc identifies group A. Penicillin is useless as a differentiating test because all streptococci remain sensitive to it, which is fortunate therapeutically but unhelpful diagnostically.

Answer: D

Correct answer: Streptococcus mutans

S. mutans is a viridans streptococcus and a normal inhabitant of the mouth, colonising the tooth surface and becoming pathogenic only when a sucrose-rich diet lets it produce the glucan and acid of dental caries. The staphylococci listed are skin flora, and S. agalactiae belongs to the genital and gastrointestinal tract.

Answer: A

Correct answer: Enterococcus faecalis

E. faecalis is a normal inhabitant of the human large intestine, which is the origin of its name and of most of the infections it causes - urinary tract infection, intra-abdominal sepsis and endocarditis all follow escape from the bowel. S. mutans belongs to the mouth, and the other two to the skin.

Answer: C

Correct answer: The morphological characteristics resemble that of S. pyogenes

This is the least accurate statement. Enterococci appear as gram-positive cocci in pairs and short chains and are usually non-haemolytic or alpha-haemolytic, whereas S. pyogenes forms long chains and is beta-haemolytic. The other three are correct: enterococci are facultative, grow in 6.5 per cent salt, and are among the commonest causes of hospital-acquired infection.

Answer: C

Correct answer: Streptococcus pyogenes

Group A streptococcus is the commonest bacterial cause of pharyngitis and the one that matters clinically, since untreated infection may be followed by rheumatic fever. S. pneumoniae causes pneumonia, otitis media and meningitis rather than pharyngitis, and Mycoplasma causes an atypical pneumonia.

Answer: C

Correct answer: Streptococcus pyogenes

Most sore throats are viral, but among bacterial causes S. pyogenes is much the commonest and the only one for which treatment is routinely given - not to shorten the illness, which is self-limiting, but to prevent the non-suppurative sequelae. Mycoplasma causes atypical pneumonia rather than pharyngitis, and staphylococcal sore throat is rare.

Answer: B

Correct answer: penicillin

Penicillin is the classical treatment for anthrax and Bacillus anthracis is usually susceptible to it. Modern guidance for inhalational and bioterrorism-associated cases begins with ciprofloxacin or doxycycline, because of the possibility of engineered resistance, with penicillin added or substituted once susceptibility is confirmed.

Answer: C

Correct answer: Both (a) and (b)

Dental caries is a plaque disease and the viridans streptococci are its principal agents. S. mutans is the best established, converting dietary sucrose to extracellular glucans that anchor the plaque and to lactic acid that demineralises enamel; S. sanguis is an early coloniser of the tooth surface and contributes to the same process. S. pyogenes is a pharyngeal and skin pathogen with no part in caries.

Answer: D

Correct answer: S. bovis

S. bovis carries the group D antigen but is not an enterococcus: it fails to grow in 6.5 per cent salt and remains bile-aesculin positive, so it is distinguished from the true enterococci by the salt test. The distinction has clinical value, since S. bovis bacteraemia and endocarditis are associated with colonic carcinoma and prompt a search for one.