Group A streptococcal bacteria originate from the throat and on the skin, while group B originates from the gastrointestinal tract and the vagina. Viridans streptococcal bacteria originates from the mouth.
There are two main classes of streptococcal bacteria: group A and group B. There is also a third group, called viridans, which can infect heart valves after passing into the bloodstream — ordinarily, it resides in the mouth.
Group A bacteria cause strep throat — they are transmitted via mucus from the nose and throat or through open skin wounds. Group B bacteria cause urinary tract infections or pneumonia. These bacteria most often infect newborns via contact with vaginal secretions. The origins of group B infections in adults are unknown, but they may come from the gastrointestinal tract of the infected.