Streptococcus Questions and Answers

Answer: C

The Gram-positive cocci associated with pneumonia are Streptococcus pneumoniae.

S. pneumoniae, the pneumococcus, appears as Gram-positive lanceolate diplococci and is the most common cause of community-acquired lobar pneumonia.

The other options fail on the Gram reaction or the shape. Neisseria are Gram-negative diplococci, and Vibrio are Gram-negative curved rods. Klebsiella pneumoniae does cause pneumonia, but it is a Gram-negative bacillus, not a coccus, so it does not fit the question.

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: B

Correct answer: Dick test

The Dick test injects a small dose of streptococcal pyrogenic (erythrogenic) exotoxin intradermally: a local erythematous reaction shows that the person lacks neutralising antibody and is therefore susceptible to scarlet fever. The Schick test is its diphtheria counterpart, and the ASO test measures antibody to streptolysin O rather than susceptibility to the toxin.

Answer: D

Correct answer: All of these

The non-suppurative sequelae are the late, immunologically mediated consequences that follow after the infection itself has resolved and no organisms remain at the site. Acute rheumatic fever follows pharyngitis, acute glomerulonephritis follows either pharyngitis or skin infection, and erythema nodosum is a recognised reactive complication. All three are distinct from the suppurative spread of the organism itself.

Answer: A

Correct answer: Cardiac valves

The group A carbohydrate of the cell wall shares antigenic determinants with glycoproteins of the cardiac valves, so antibody raised against the organism attacks the valve tissue as well. This is the molecular mimicry that produces the valvular damage of rheumatic heart disease. The M protein cross-reacts with a different tissue, the myocardium.

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: all of these

Streptolysin O is antigenic, which is why the antistreptolysin O titre can be measured and used to establish a recent infection. It is oxygen-labile, being inactivated by atmospheric oxygen, which is why the haemolysis it produces is best seen in a stab or under a coverslip. And it is heat-labile. Streptolysin S is the opposite on the first two counts: oxygen-stable and non-antigenic.

Answer: C

Correct answer: myocardium

The M protein, the chief cell wall protein and the principal virulence factor, shares epitopes with cardiac muscle, so antibody directed against it also attacks the myocardium. Together with the cell wall carbohydrate’s cross-reaction with the valves, this accounts for the pancarditis of rheumatic fever - two different bacterial antigens attacking two different cardiac tissues.

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: D

Correct answer: All of these

All three detect antibody rather than the organism, so all can establish an infection that has already resolved - which is exactly what is needed when rheumatic fever or glomerulonephritis appears weeks later. The ASO test detects antibody to streptolysin O, anti-DNase B is more reliable after skin infection, and the streptozyme test screens for several streptococcal antibodies at once.