Viridans
Summary
Viridans streptococci are alpha-haemolytic, optochin-resistant, bile-insoluble gram-positive cocci that are normal oral flora. They are the leading cause of subacute bacterial endocarditis on previously damaged valves and are central to dental caries.
Detail
The name comes from the green (viridis) discolouration of alpha-haemolysis on blood agar. Their defining laboratory features exist to separate them from Streptococcus pneumoniae, which shares alpha-haemolysis but is optochin-sensitive, bile-soluble, and encapsulated. Species include S. mutans and S. sanguinis in the mouth, S. mitis, and the S. anginosus (milleri) group, which characteristically forms abscesses in brain, liver, and abdomen. S. mutans metabolizes dietary sucrose to produce dextrans that let it adhere to enamel and generate acid, causing caries. After dental procedures or with poor dentition, transient bacteraemia allows viridans streptococci to adhere via dextran to damaged or prosthetic valves, producing subacute endocarditis with an indolent course, low-grade fever, and immune phenomena such as Osler nodes, Roth spots, and glomerulonephritis. Prophylaxis with amoxicillin before dental procedures is now restricted to the highest-risk patients: prosthetic valves, previous endocarditis, certain congenital heart disease, and cardiac transplant valvulopathy.
Sources
- Levinson Medical Microbiology and Immunology
- First Aid for the USMLE Step 1
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