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The viridans streptococci are a large group of commensal streptococcal bacteria species that are either α-hemolytic, producing a green coloration on blood agar plates (hence the name "viridans", from Latin "vĭrĭdis", green), or nonhemolytic. The pseudotaxonomic non-Linnean term Streptococcus viridans is often used to refer to this group of species, but writers that do not like to use the pseudotaxonomic term (which treats a group of species as if they were one species) prefer the terms viridans streptococci or viridans streptococcal species.
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|
The organisms are most abundant in the mouth, and one member of the group, S. mutans, is the etiologic agent of dental caries in most cases and populations. S. sanguinis is also another potential cause. Others may be involved in other mouth or gingival infections as pericoronitis.
If they are introduced into the bloodstream, they have the potential of causing endocarditis, in particular 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 dental extraction).
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