Group B Streptococcus (GBS)

GBS is a gram-positive coccus (round bacterium) with a tendency to form chains (as reflected by the genus name Streptococcus). It is a beta-hemolytic, catalase-negative, and facultative anaerobe. In general, GBS is a harmless commensal bacterium being part of the human microbiota colonizing the gastrointestinal and genitourinary tract of up to 30% of healthy human adults (asymptomatic carriers). Nevertheless, GBS can cause severe invasive infections. Streptococcus agalactiae is the species designation for streptococci belonging to group B of the Lancefield classification. GBS is surrounded by a bacterial capsule composed of polysaccharides (exopolysacharide). The species is subclassified into ten serotypes (Ia, Ib, II–IX) depending on the immunologic reactivity of their polysaccharide capsule.