Viridans Streptococcus is a pseudotaxonomic non-Linnaenan term for a large group of commensal streptococcal

Streptococcus

Streptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning...

 bacteria that are either α-hemolytic

Hemolysis (microbiology)

Hemolysis is the breakdown of red blood cells. The ability of bacterial colonies to induce hemolysis when grown on blood agar is used to classify certain microorganisms. This is particularly useful in classifying streptococcal species...

, 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

Streptococcus pneumoniae

Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, aerotolerant anaerobic member of the genus Streptococcus. A significant human pathogenic bacterium, S...

 using an optochin

Optochin

Optochin is a chemical used in cell culture techniques for the presumptive identification of Streptococcus pneumoniae, which is optochin-sensitive, from other alpha-hemolytic streptococci such as Streptococcus viridans which are resistant...

 test, as Viridans streptococci are optochin resistant; they also lack either the polysaccharide

Polysaccharide

Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...

-based capsule typical of S.