Causes of community-acquired native valve endocarditis are

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 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). Reference: Textbook of Microbiology; Anathanarayan and paniker's; 10th edition; Page no: 220