Streptococcus Questions and Answers

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: Staphylococcus aureus

The walled-off abscess is the characteristic staphylococcal lesion: coagulase clots the surrounding plasma into a fibrin barrier that keeps phagocytes and antibiotics out while the organism multiplies inside. Streptococcal infection spreads diffusely instead, through tissue planes, because streptokinase and hyaluronidase break down exactly the barriers that staphylococci build.

Answer: D

Correct answer: It grows well in a high concentration of glucose media

Pneumococci are fastidious and are inhibited rather than favoured by a high glucose concentration, because the acid produced from fermenting it lowers the pH below what the organism tolerates. The other three are all correct: it is a capsulated diplococcus, it is alpha-haemolytic on blood agar, and it is carried in the nasopharynx of healthy people.

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: A

Correct answer: Meningitis

Meningitis is not a typical group A streptococcal infection; it is caused chiefly by S. pneumoniae, Neisseria meningitidis and, in the newborn, group B streptococcus. The other three are all group A diseases: pharyngitis, impetigo, and scarlet fever, which is pharyngitis together with the rash produced by the pyrogenic exotoxin.

Answer: A

Correct answer: Endotoxins

Endotoxin is lipopolysaccharide, a component of the gram-negative outer membrane, and S. pyogenes is gram-positive and has none. Its virulence rests on the M protein, which resists phagocytosis and is the principal antigen; the streptolysins, which lyse cells; and enzymes such as streptokinase and hyaluronidase, which spread the infection through tissue.

Answer: B

Correct answer: False

Scarlet fever is streptococcal pharyngitis plus the effects of the pyrogenic exotoxin: a diffuse erythematous rash sparing the area around the mouth, a strawberry tongue, and subsequent desquamation. It occurs only when the infecting strain carries the toxin gene and the patient lacks neutralising antibody, which is what the Dick test detects.

Answer: C

Correct answer: uterus

Puerperal fever is streptococcal infection of the endometrium following childbirth, spreading from there to the bloodstream. It was the great killer of maternity wards until Semmelweis showed that hand disinfection prevented it, and it remains the historical demonstration that the organism is carried on the hands of attendants rather than arising within the patient.

Answer: B

Correct answer: Staphylococcus aureus

Impetigo is caused by S. aureus and by S. pyogenes, and S. aureus is now the commoner of the two in most settings, producing the bullous form as well as the crusted one. Both are gram-positive cocci, which is as far as the microscopy in the case takes you; culture and coagulase testing separate them.

Answer: A

Correct answer: humoral immunity

Protection against S. pyogenes is antibody-mediated and directed against the M protein, and it is type specific: antibody to one M type protects against that type alone, which is why repeated infections with different types are possible throughout life. The type specificity is also the chief obstacle to a vaccine, since there are more than a hundred M types.