Viridans streptococci are an ill defined group of species which typically show alpha haemolysis on blood agar, but may have a variety or no Lancefield antigens. The most common are Str. sanguis, mitior, mutans and salivarius. Most are commensals of the mouth and upper respiratory tract.
Diseases caused include:
- endocarditis on abnormal valves - especially Str. sanguis
- gingivitis, dental caries - especially Str. mutans
They are sensitive to penicillin (most strains) and erythromycin. Tetracyclines, cephalosporins and co - trimoxazole are less effective.