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Strep viridans is collective term for a group of Streptococcus species with alpha-haemolytic activity. This produces a green coloration around the colonies when grown on blood agar culture, hence the name viridans (Latin for green).
Strep viridans species commonly live around the teeth. Bacteria are dislodged into the circulation by movement of those teeth, but this includes ordinary use and cleaning of them as well as dental scaling and operations.
Organisms under the Strep viridans umbrella have been found growing on people's endocardium, sometimes people who died of it.
People with heart murmurs (indicating turbulence at resting cardiac output), known malformations of the heart, or history of rheumatic fever or scarlet fever have been held to be more prone to developing infective endocarditis, and have been treated against this possibility with a bolus dose of an antibiotic (eg amoxycillin 3 grams) shortly before dentistry.