Streptococcus viridans is a group which normally resides in the mouth and upper respiratory tract and is alpha hemolytic. In persons with pre-existing cardiac lesions, they may cause bacterial endocarditis, Streptococcus sanguis being most often responsible. Following the tooth extraction or dental procedures, they cause transient bacteremia and get implanted on damaged or prosthetic valves or in congenitally diseased hea and grow to form vegetation. Streptococcus mutans is also a member of viridans group which causes endocarditis in individuals with risk factors ( dental extraction in people with damaged valves). The bacterium has a polysaccharide coat that allows it to stick to teeth and also to damaged hea valves, it can invade the bloodstream. Reference: Textbook of Microbiology; Anathanarayan and panicker's; 10th edition; Page no: 220, 221