The viridans streptococci are a large group of commensal streptococcal Gram-positive bacteria species that are α-hemolytic, producing a green coloration on blood agar plates (hence the name "viridans", from Latin "vĭrĭdis", green), although some species in this group are actually γ-hemolytic, meaning they produce no change on blood agar.

[5] The organisms are most abundant in the mouth, and one member of the group, S. mutans, is the cause of dental caries in most cases and populations.

[6] Viridans streptococci have the unique ability to synthesize dextrans from glucose, which allows them to adhere to fibrin-platelet aggregates at damaged heart valves.

This mechanism underlies their ability to cause subacute valvular heart disease following their introduction into the bloodstream (e.g., following dental extraction).

[7] Naveen Kumar, Venkatesan; van der Linden, Mark; Menon, Thangam; Patric Nitsche-Schmitz, D. (May 2014).