A large heterogeneous group of mostly alpha-hemolytic streptococci. They colonize the respiratory tract at birth and generally have a low degree of pathogenicity. This group of species includes streptococcus mitis; streptococcus mutans; streptococcus oralis; streptococcus sanguis; streptococcus sobrinus; and the streptococcus milleri group. The latter are often beta-hemolytic and commonly produce invasive pyogenic infections including brain and abdominal abscesses.