Description
Amyl Group B Strep. Agar (Islam), AM 809, is formulated according to Islam1, who designed the medium in order to capitalise on the ability of the majority of strains of βhaemolytic streptococci of Lancefield’s Group B, (Streptococcus agalactiae), isolated from human infections, to produce red/orange pigmented colonies when incubated anaerobically on starch containing media.
Principles and Explanation: Streptococci of Lancefield’s Group B, (GBS), are the causative organisms of a significant proportion of neonatal sepsis acquired from the genital tract of colonised mothers during the birthing process. The infections are manifested as bacterial meningitis and septicaemia, with a high mortality rate in delayed diagnosis and treatment. A substantial number of cases of early onset GBS disease can be prevented by administration of appropriate prophylactic antimicrobial agents during labour. Prospective mothers are normally screened for the presence of colonising GBS in the genital or rectal areas during the third trimester of pregnancy (35-37 weeks), by screening specimens obtained from the lower rectum and vagina.
GBS isolated from human infections are often haemolytic and pigmented, a linkage that is not reflected in bovine isolates2. Lancefield3, first noted the pigmentation of GBS isolates in 9 of 24 strains when incubated anaerobically, a proportion that has increased with improvements in culture media, to between 95 and 97% of haemolytic strains4. Further work performed by de la Rosa and colleagues5, demonstrated the enhancement of pigment production by GBS after incorporating cotrimoxazole into the culture medium, an effect that can be replicated by placing a sensitivity disc containing sulphonamides onto the surface of an inoculated plate and observing enhanced pigmentation of colonies closest to the disc.
