Streptococcus mutans is facultatively anaerobic, Gram-positive coccus-shaped bacterium commonly found in the human oral cavity and is a significant contributor to tooth decay. The microbe was first described by J Kilian Clarke in 1924.
Two closely related species, Streptococcus mutans and Streptococcus sobrinus, can cohabit the mouth, both contributing to oral disease, and the expense of differentiating them in laboratory testing is often not clinically necessary. Therefore, for clinical purposes they are sometimes considered together as a group, called the mutans streptococci. link to wiki