Streptococcus mutans is a Gram positive streptococcus bacterium that inhabits human teeth and has been demonstrated to be the primary bacterial cause of dental caries (commonly known as cavities). S. mutans is often part of the normal flora or normal microbiota of humans, so the presence of S. mutans in the mouth does not automatically cause dental caries. However, S. mutans can be transmitted among humans, so caries is classified as an infectious disease.
Here is a culture of Streptococcus mutans grown in the laboratory: