Streptococcus mutans is a facultatively anaerobic, gram-positive coccus (round bacterium) commonly found in the human oral cavity and is a significant contributor to tooth decay. It is part of the "streptococci" (plural, non-italic lowercase), an informal general name for all species in the genus Streptococcus.The microbe was first described by J Kilian Clarke in 1924.
This bacterium, along with the closely related species Streptococcus sobrinus, can cohabit the mouth: Both contribute to oral disease, and the expense of differentiating them in laboratory testing is often not clinically necessary. Therefore, for clinical purposes they are often considered together as a group, called the mutans streptococci (plural, non-italic due to it being an informal group name). This grouping of similar bacteria with similar tropism can also be seen in the viridans streptococci, another group of Streptococcus species.