viridansGruppe

The viridans group streptococci, often abbreviated as VGS, are a large and heterogeneous collection of Gram-positive cocci within the genus Streptococcus. They are typically alpha-hemolytic on blood agar, yielding a greenish discoloration, but some strains are non-hemolytic. They are facultative anaerobes and are catalase negative. A key laboratory distinction from Streptococcus pneumoniae is that most viridans streptococci are optochin resistant and bile insoluble.

Ecology and clinical relevance

Viridans streptococci are common constituents of the normal human microbiota, particularly in the oral cavity, as well as the upper respiratory, gastrointestinal, and genitourinary tracts. They are associated with dental plaque and contribute to dental caries and other oral infections. Although usually harmless, they can become opportunistic pathogens. The most clinically significant infection is subacute bacterial endocarditis, especially after dental procedures in patients with preexisting valvular heart disease or prior valve damage. They can also cause bacteremia, brain abscesses, and, less frequently, intrabdominal or deep-seated infections.

Taxonomy and subgroups

The viridans group is not a single taxonomic unit but a collection of several subgroups, including the Mitis group (for example, Streptococcus mitis, S. oralis, S. sanguinis), the Anginosus group (S. anginosus, S. constellatus, S. intermedius), the Mutans group (S. mutans, S. sobrinus), the Salivarius group (S. salivarius), and the Bovis group (S. gallolyticus, formerly S. bovis). The Bovis group has been linked to colon cancer in some contexts, warranting clinical attention.

Diagnosis and treatment

Identification relies on morphology, hemolysis pattern, optochin susceptibility (optochin resistant), bile insolubility, and often MALDI-TOF or molecular methods. Antibiotic susceptibility is typically penicillin-susceptible, but resistance can occur. Treatment of endocarditis or invasive infections generally begins with penicillin or ampicillin, with adjunctive agents (such as gentamicin) as indicated; vancomycin is used for resistant strains or penicillin-allergic patients.