This organism stains positive on Gram stain due to thick peptidoglycan layer which absorbs crystal violet.
This bacterium has a spherical shape.
Characteristically, Streptococcus viridans is catalase-negative, which is helpful in distinguishing Streptococcus from Staphylococcal species that are catalase- positive.
This bacteria is alpha-hemolytic, which causes dark green colonies on blood agar. It is caused by hydrogen peroxide produced by bacterium, which oxidizes hemoglobin to green methemoglobin. Strep viridans may also be non-hemolytic (sometimes termed γ-hemolysis).
Optochin test aids in the differentiation between Streptococcus pneumoniae and Streptococcus viridans. Streptococcus viridans is optochin-resistant, meaning the growth of bacteria is not inhibited around an optochin disc, unlike Streptococcus pneumonia, which is optochin-sensitive.
Streptococcus viridans can be differentiated from Streptococcus pneumoniae based on sensitivity to lysis by bile. Streptococcus pneumonia is bile soluble and will lyse in presence of bile, while Streptococcus viridans will not.
The organisms are most abundant in the mouth as normal flora.
Certain strains of this bacteria (S. Sanguis and S. Mutans) may produce extracellular polysaccharides, also known as dextrans, by using sucrose as a substrate. Dextrans promote adherence of streptococcal bacteria to fibrin which is a factor of causing subacute bacterial endocarditis.
S. mutans is associated with dental caries.
If organisms are introduced into the bloodstream, they have the capacity to cause endocarditis, especially in individuals with previously damaged heart valves, due to a unique ability to synthesize dextrans. The dextrans allow the organisms to adhere to fibrin-platelet aggregates present on damaged heart valves.
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