Streptococcus viridans

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Streptococcus viridans-group
File:Streptococcus viridans 01.png
Scientific classification
Kingdom: Eubacteria
Phylum: Firmicutes
Class: Bacilli
Order: Lactobacillales
Family: Streptococcaceae
Genus: Streptococcus

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Streptococcus viridans is a pseudo-taxonomic term for a large group of generally non-pathogenic, commensal streptococcal bacteria that are either α-hemolytic and produce a green (hence the name viridans) coloration on blood agar plates, or non-hemolytic.[1]


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.[2]


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.


  1. Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed. ed.). McGraw Hill. pp. 293–4. ISBN 0838585299.
  2. Patterson MJ (1996). "Streptococcus". Baron's Medical Microbiology (Baron S et al, eds.) (4th ed. ed.). Univ of Texas Medical Branch. ISBN 0-9631172-1-1.
  3. "Public Health Image Library (PHIL)".


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