Colonies of group A beta-hemolytic streptococci grown on blood agar. Clearly visible the beta-hemolytic zone.
Group A streptococci produce streptolysins. Streptolysin S causes the hemolytic effect when cultured on blodd agar. Streptolysin O causes a measurable antibody response (ASO titre) indicative of recent infection.
Pyrogenic exotoxins A, B and C are superantigens and activate MHC class II molecules directly, without antigen presentation. The exotoxins are responsible for scarlet fever and STSS (streptococcal toxic shock syndrome).
Group A beta-hemolytic streptococci are the causative agent of:
- pharyngitis
- scarlet fever (children > 3 year)
- pneumonia
- cellulitis
- impetigo
- necrotising fasciitis
- streptococcal toxic shock syndrome
- and its sequelae:
- rheumatic fever
- post streptococcal glomerulonephritis (types 12 and 49)
- Sydenham chorea
Therapy
All beta-lactams except aztreonam are effective, vancomycin or linezolid may be alternatively used. Macrolides, clindamycin and doxycyclin are effective as well. Moxifloxacin should be effective but not levofloxacin or ciprofloxacin. Clindamycin inhibits toxin production and can be added to betalactam antibiotics in severe disease.
For necrotising fasciitis with streptococcal toxic shock syndrome, the primary therapy consists of surgery.
In septic patients, intravenous immunoglobulin G can be an effective adjunctive therapy.
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