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Colonies of viridans streptococci on blood agar surroundend by a wide zone of alpha-hemolysis. Cultivation 24 hours in an aerobic
atmosphere enriched with 5% carbon dioxide, 37°C.
Viridans Streptococcus is a pseudotaxonomic non-Linnaenan term for a large group of commensal streptococcal bacteria that are either alpha-hemolytic,
producing a green coloration on blood agar plates (hence the name "viridans", from Latin "viridis", green), or nonhemolytic. They possess no Lancefield antigens.
In general, pathogenicity is low. The organisms are most abundant in the mouth, and one member of the group, S. mutans, is the etiologic agent of dental caries.
Others may be involved in other mouth or gingival infections. If they are introduced into the bloodstream, they have the potential of causing endocarditis, particularly in
individuals with damaged heart valves. They are the most common causes of subacute bacterial endocarditis.
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 tooth extraction).
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