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. The microbe was first described by James 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. This grouping of similar bacteria with similar tropism can also be seen in the viridans streptococci – of which Streptococcus mutans is itself also a member.

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