viridansStreptokokken

Viridans streptokokken, or viridansStreptokokken, refers to a heterogeneous group of Gram-positive, catalase-negative cocci within the genus Streptococcus. They are characterized by a greenish discoloration on blood agar due to partial alpha-hemolysis (alpha-hemolysis) or by non-hemolysis, and they lack a single, consistent Lancefield antigen, so they are not divided into a uniform serogroup. The group includes several species, notably Streptococcus mitis, S. mutans, S. sanguinis, S. oralis, S. salivarius, and the Streptococcus anginosus (milleri) group.

Ecology and clinical relevance: Viridans streptococci are common residents of the human oral cavity, oropharynx, and upper respiratory tract, with additional presence in the gastrointestinal and female genital tracts. They are associated with dental biofilms and caries (especially S. mutans) and are a major cause of infective endocarditis, particularly after dental procedures or in individuals with preexisting heart valve abnormalities. They can also cause bacteremia and various localized infections, especially in immunocompromised patients.

Identification and laboratory features: They appear as gram-positive cocci in pairs or short chains, are catalase-negative, and are typically optochin-resistant and bile-insoluble. Hemolysis on blood agar is variable (often alpha- or gamma-hemolysis). Due to species diversity and variable susceptibility, laboratory identification and antibiotic testing often require a combination of phenotypic methods and, when needed, molecular approaches.

Treatment considerations: Many viridans streptococci are penicillin-susceptible, but resistance occurs in some strains. Therapy is guided by susceptibility testing and may include penicillin or ceftriaxone; penicillin-non-susceptible isolates may require alternative agents such as vancomycin. Endocarditis regimens may involve combination therapy and are tailored to the specific susceptibility profile.