GBS Agar Plate

Principle And Interpretation

Beta-haemolytic Streptococci with Lancefield group B antigen (Streptococcus agalactiae) are an important cause of serious neonatal infection characterized by sepsis and meningitis. Heavy colonization of the maternal genital tract is associated with colonization of infants and risk of neonatal disease (6). GBS Medium, formulated by Islam (1,2,5) is recommended for the isolation and detection of group B Streptococci (GBS) from clinical specimens. GBS Medium is designed to exploit the ability of most Group B Streptococci (GBS) to produce orange/red pigmented colonies when incubated under anaerobic conditions. The orange red pigment of group B Streptococci also has the characteristic of a carotenoid (5). GBS Medium Base also supports growth of other genital bacteria that cause perinatal infections (6), e.g. anaerobic Streptococcus, Bacteroides and Clostridium species.

Proteose peptone provides the necessary nutrients for the growth of Group B Streptococci. The phosphate salts buffer the medium. The antibiotic Metronidazole and Gentamicin makes the medium selective for Group B Streptococci, while the horse serum enriches the medium. Species such as Gardenerella vaginilis gets inhibited by Metronidazole. Anaerobic bacteria such as fusobacteria, and clostridia gets inhibited by metrinidazole.

Vaginal or rectal swabs should be inserted vertically into the medium. Incubation is carried out at 35-37°C. Pigment production is observed at hourly interval. Presence of blood in the specimen may give false positive results. Presumptively positive tubes should be further confirmed by biochemical analysis to identify Group B Streptococci.