Groups identified by Rebecca Lancefield. Group B is seen in the genitourinary tract of women and is an important pathogen in neonates. Groups C and G both cause invasive disease, both have M proteins, more common throat carriage in developing countries. Maybe also responsible for acute rheumatic fever. Alpha-haemolytic (strep viridans) are associated with line infections and endocarditis. Gamma-haemolytic include enterococci, of which faecium is usually resistant to amoxicillin but faecalis is usually sensitive. Of note, some enterococci are now resistant to vancomycin (VRE).
M type proteins on GAS are the main virulence factors esp 1, 3. About 20 toxins, all superantigens. Antibodies to toxin are effective. Some HLA types appear more susceptible. IVIG effective against toxins. Clindamycin may have better effect against toxin production.
Clinical
- Erysipelas – rapidly spreading skin infection with clear demarcation, can evolve over a few hours. Differential is staphylococcus.
- Necrotizing fasciitis
- Sepsis. Group A strep sepsis is provoked by varicella. A quarter of all invasive strep infections in kids follow chickenpox esp toxic shock, osteomyelitis. ?Shifts T helper cells from Th2 (antibody) to Th1 (cellular).
Group A streptococcus has never been resistant to penicillin! But doesn’t mean penicillin is the best treatment: clindamycin shows superior killing (in animal models) – inhibits protein synthesis so works throughout cell cycle, cf penicillin where action only during replication. Will also inhibit toxins (in theory).
GAS vaccine? – danger of priming for rheumatic fever! New 26 valent, effective for sore throats and invasive disease, probably not long term effective or appropriate for preventing rheumatic fever.