Viridans

Viridans streptococci, or viridans group streptococci (VGS), are a diverse collection of gram-positive cocci within the genus Streptococcus. They are typically alpha-hemolytic or non-hemolytic on blood agar, catalase-negative, optochin-resistant, and bile-insoluble. They are not encapsulated and do not belong to a single Lancefield group.

Ecology and disease: Viridans streptococci are common inhabitants of the human oral cavity, oropharynx, and gastrointestinal tract. Several species produce extracellular glucans from sucrose that facilitate adherence to tooth surfaces, contributing to dental plaque and dental caries; notably S. mutans and S. sobrinus are associated with caries. Although normally harmless, viridans streptococci can enter the bloodstream during dental procedures or mucosal trauma and cause infections such as subacute endocarditis, brain abscesses, otitis media, or sinusitis, particularly in individuals with heart valve disease or immunocompromise.

Diagnosis and treatment: Laboratory identification relies on Gram stain showing gram-positive cocci in pairs or short chains and the described hemolysis and growth characteristics (alpha- or gamma-hemolysis; optochin resistant; bile insoluble). Antibiotic susceptibility is variable; penicillin or ampicillin is commonly used, sometimes with an aminoglycoside for endocarditis. Resistance to macrolides and tetracyclines occurs in some strains. Prophylaxis for dental procedures is advised for certain high-risk patients, typically with amoxicillin before procedures; alternatives exist for penicillin allergy according to guidelines.

Taxonomy and species: The viridans group includes species such as S. mitis, S. oralis, S. sanguinis, S. mutans, S. sobrinus, S. anginosus (with S. constellatus and S. intermedius in the anginosus group), and S. salivarius, among others. They are part of the normal flora of the mouth and upper respiratory tract.