Viridans streptococci

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Viridans streptococci
Streptococcus viridans 01.png
Scientific classification
Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Lactobacillales
Family: Streptococcaceae
Genus: Streptococcus

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 pseudo-taxonomic 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[1] or viridans streptococcal species.

These species possess no Lancefield antigens.[2] In general, pathogenicity is low.[3]


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.[4]

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


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).


  1. ^ Dorland's Illustrated Medical Dictionary, headword "streptococcus", subentry "viridans streptococci".
  2. ^ Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 293–4. ISBN 0-8385-8529-9. 
  3. ^ Viridans Streptococci at the US National Library of Medicine Medical Subject Headings (MeSH)
  4. ^ 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. 

Further Reading[edit]

Naveen Kumar, Venkatesan; van der Linden, Mark; Menon, Thangam; Patric Nitsche-Schmitz, D. (May 2014). "Viridans and bovis group streptococci that cause infective endocarditis in two regions with contrasting epidemiology". International Journal of Medical Microbiology 304 (3-4): 262–268. doi:10.1016/j.ijmm.2013.10.004. Retrieved 31 October 2014.