Streptococcus viridans

Streptococcus viridans-group
Scientific classification
Kingdom:	Bacteria
Phylum:	Firmicutes
Class:	Cocci
Order:	Lactobacillales
Family:	Streptococcaceae
Genus:	Streptococcus

Viridans Streptococcus is a pseudotaxonomic non-Linnaenan term for a large group of commensal streptococcal bacteria that are either α-hemolytic, producing a green coloration on blood agar plates (hence the name "viridans", from Latin "vĭrĭdis", green), or nonhemolytic. They possess no Lancefield antigens.

In general, pathogenicity is low.

Identification

Viridans streptococci can be differentiated from Streptococcus pneumoniae using an optochin test, as Viridans streptococci are optochin resistant; they also lack either the polysaccharide-based capsule typical of S. pneumoniae or the Lancefield antigens of the pyogenic members of the genus.

	Viridans streptococci	Streptococcus pneumoniae
Solubility in bile	Insoluble	Soluble
Fermentation of inulin	Not a fermenter	Fermenter with acid production
Sensitivity to optochin	Not sensitive	Sensitive
Pathogenicity to mice	Nonpathogenic	Pathogenic
Quellung test	Negative	Positive

Pathology

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).

Treatment

• See List of antibiotics.