Viridans Streptococci is a pseudotaxonomic non-Linnean 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.
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. 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 dental extraction).
- See List of antibiotics.
- Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 293–4. ISBN 0-8385-8529-9.
- Viridans Streptococci at the US National Library of Medicine Medical Subject Headings (MeSH)
- Patterson MJ (1996). "Streptococcus". Baron's Medical Microbiology (Baron S et al., eds.) (4th ed.). Univ of Texas Medical Branch. ISBN 0-9631172-1-1.
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