Group B Streptococcus. Group B Streptococcus is a bacterial infection of Streptococcus agalactiae. It is a facultative anaerobic bacteria that is gram positive with a cocci morphology found in chains. Many GBS strains are mainly encapsulated and are then serologically classified according to the presence of type-specific capsular polysaccharides. These capsules are important virulence factors and stimulators of antibody-associated immunity..