Overview: Streptococci are among the most abundant bacteria in the mouth, making up 30-60% of the bacteria on the buccal mucosa and 15-30% of the bacteria on the tongue dorsum and in supragingival plaque.

Ecological role/importance in health and disease: The 37 species of Streptococcus listed in HOMD include many members of the healthy commensal microbiota as well as several that are pathogens and one that is used to produce yogurt (S. thermophilus). Streptococci such as S. mitis and S. gordonii are among the first bacteria to colonize the tooth surface after it is cleaned, where they begin the process of ecological succession (Diaz et al. 2006). S. salivarius is abundant on the tongue dorsum and can carry out nitrate reduction to nitrite, thereby participating in the physiology of its human host (Hezel and Weitzberg 2013). Other streptococci that are usually rare in the healthy mouth can cause disease, such as S. mutans, which is associated with dental caries, S. pyogenes, which causes strep throat, and S. pneumoniae, which can cause pneumonia. In the subgingival plaque biofilm, most streptococci are grouped into the "yellow complex", but S. constellatus is a member of the "orange complex" marking part of the ecological succession from health toward periodontitis (Socransky et al. 1998).