Viridans streptococci

Viridans streptococci. They are a large, heterogeneous group of Gram-positive, alpha-hemolytic or non-hemolytic streptococci that are commensal inhabitants of the human oral cavity, gastrointestinal tract, and female genital tract. While typically part of the normal flora, they are opportunistic pathogens capable of causing serious infections, most notably infective endocarditis, particularly following dental procedures or in patients with prosthetic heart valves. Their classification has evolved significantly with the advent of 16S rRNA sequencing and whole-genome analysis, leading to the reclassification of many species into new genera such as Abiotrophia and Granulicatella.

Classification and taxonomy

The taxonomy of this group is complex and has been revised extensively through modern molecular techniques. Historically, they were categorized based on hemolytic patterns on blood agar, but this has been superseded by genetic analysis. Key species include the ''Streptococcus mitis'' group, the ''Streptococcus anginosus'' group (also known as the ''Streptococcus milleri'' group), the ''Streptococcus salivarius'' group, the ''Streptococcus mutans'' group, and the ''Streptococcus bovis'' group, the latter now largely reclassified within the genus ''Streptococcus gallolyticus''. The ''mitis'' group includes common oral commensals like ''Streptococcus oralis'' and ''Streptococcus sanguinis'', which are frequent agents of subacute bacterial endocarditis. The ''anginosus'' group is notable for its propensity to cause pyogenic infections such as brain and liver abscesses.

Pathogenesis and clinical significance

Their clinical significance stems from their ability to cause disease when they gain access to sterile sites. The most classic association is with infective endocarditis, where they adhere to damaged heart valves or endocardial tissue, forming vegetations. This process is facilitated by bacterial surface proteins that bind to host extracellular matrix proteins like fibrinogen and fibronectin. Beyond endocarditis, they are implicated in dental caries (primarily the ''mutans'' group, including ''Streptococcus sobrinus''), sinusitis, pneumonia, meningitis, and bacteremia, particularly in immunocompromised patients such as those undergoing chemotherapy or with neutropenia. The ''Abiotrophia defectiva'' species, formerly known as nutritionally variant streptococci, is especially notorious for causing treatment-refractory endocarditis.

Identification and laboratory characteristics

In the clinical microbiology laboratory, they are identified by their characteristic alpha-hemolytic (green) or non-hemolytic reaction on sheep blood agar, negative catalase test, and Gram-positive cocci in chains. They are distinguished from the pneumococcus (Streptococcus pneumoniae) by their resistance to optochin and lack of bile solubility. Further speciation often requires biochemical profiling using systems like the API 20 Strep or, increasingly, MALDI-TOF mass spectrometry and PCR-based methods. The Voges–Proskauer test and arginine hydrolysis are among the traditional tests used to differentiate groups like the ''salivarius'' and ''anginosus'' groups.

Epidemiology and transmission

These bacteria are ubiquitous components of the human microbiota from early infancy, primarily colonizing the oropharynx and gastrointestinal tract. Transmission occurs through direct contact with oral secretions or via aerosols. The risk of invasive disease, such as endocarditis, increases following dental extractions, periodontal surgery, or other invasive procedures that cause transient bacteremia. Populations at highest risk include individuals with congenital heart disease, rheumatic heart disease, prosthetic valves, or a history of intravenous drug use. The American Heart Association and the European Society of Cardiology issue guidelines for antibiotic prophylaxis in high-risk patients undergoing such procedures.

Treatment and antimicrobial resistance

Treatment depends on the infection site and the specific species involved, guided by antimicrobial susceptibility testing. For endocarditis, prolonged therapy with penicillin G or ceftriaxone, often combined with an aminoglycoside like gentamicin for synergy, is standard. However, rising rates of antimicrobial resistance are a concern. Many isolates now exhibit decreased susceptibility to penicillin, and resistance to macrolides (e.g., erythromycin), tetracyclines, and fluoroquinolones is increasingly reported. Vancomycin remains a reliable alternative for penicillin-allergic patients or infections with multidrug-resistant strains. The Clinical and Laboratory Standards Institute provides breakpoints for interpreting susceptibility results for these organisms|Clinical and Laboratory and Laboratory Standards Institute and the Clinical and the Clinical and Laboratory Standards Institute provides Clinical and Laboratory Standards Institute and Laboratory Standards Institute provides the Laboratory Standards Institute and Laboratory Standards Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute Institute.

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