Streptococcus agalactiae (also known as group B streptococcus or
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.
S. agalactiae is the most common human pathogen of streptococci belonging
to group B of the Rebecca Lancefield classification of streptococci. GBS are
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.