Viridans and Nonhemolytic Streptococci
The viridans group comprises all β-hemolytic streptococci that remain after the criteria for defining pyogenic streptococci and pneumococci have been applied. Characteristically members of the normal flora of the oral and nasopharyngeal cavities, they have the basic bacteriologic features of streptococci but lack the specific antigens, toxins, and virulence of the other groups. Although the viridans group includes many species (see Table 17 – 2), they are usually not completely identified in clinical practice because there is little differ-ence among them in medical significance.
Although their virulence is very low, viridans strains can cause disease when they are protected from host defenses. The prime example is subacute bacterial endocarditis. In this disease, viridans streptococci reach previously damaged heart valves as a result of transient bacteremia associated with manipulations, such as tooth extraction, that disturb their usual habitat. Protected by fibrin and platelets, they multiply on the valve, causing local and sys-temic disease that is fatal if untreated. Extracellular production of glucans, complex poly-saccharide polymers, may enhance their attachment to cardiac valves in a manner similar to the pathogenesis of dental caries by S. mutans . The clinical course of viri-dans streptococcal endocarditis is subacute, with slow progression over weeks or months . It is effectively treated with penicillin, but uniformly fatal if untreated.
The disease is particularly associated with valves damaged by recurrent rheumatic fever. The decline in the occurrence of rheumatic heart disease has reduced the incidence of this particular type of endocarditis.