Strep viridans

This is a group rather than a single species, comprising Streptococcus mutans, sanguinis, mitis, salivarius, and the anginosus group. All are catalase-negative gram-positive cocci in chains, alpha-haemolytic on blood agar, and are separated from Streptococcus pneumoniae by being optochin-resistant and insoluble in bile. Two clinical roles dominate. In the mouth, S. mutans converts sucrose to extracellular dextran polymers, adhering to enamel and generating lactic acid that demineralizes it, causing caries. In the circulation, dextran-mediated adherence lets the organism colonize valves already damaged by rheumatic disease, mitral valve prolapse with regurgitation, congenital lesions, or prosthetic material, producing subacute endocarditis with small vegetations, weeks of low-grade fever, weight loss, new or changed murmur, and immunological phenomena. Blood cultures should be drawn from separate sites before antibiotics, and treatment is penicillin or ceftriaxone with or without gentamicin. The anginosus group is exceptional in forming deep abscesses rather than endocarditis.