What are the differences between Streptococcus viridans and Streptococcus salivarius in terms of clinical significance, laboratory identification, and recommended antimicrobial therapy?

Streptococcus viridans vs Streptococcus salivarius: Key Differences

Streptococcus salivarius is a specific species within the broader viridans group streptococci, not a separate entity—understanding this taxonomic relationship is essential for proper clinical management and laboratory identification. 1

Taxonomic Classification

• Viridans group streptococci (VGS) is an umbrella term for α-hemolytic streptococci that includes multiple species: S. sanguis, S. oralis (mitis), S. salivarius, S. mutans, and the S. anginosus group (S. intermedius, anginosus, constellatus). 1

• S. salivarius is one specific species within this viridans group, classified within the "Salivarius group" along with S. vestibularis and S. thermophilus. 2, 3

• All viridans streptococci, including S. salivarius, are gram-positive cocci that produce α-hemolysis on blood agar, are optochin-resistant, and bile-insoluble (distinguishing them from S. pneumoniae). 3

Laboratory Identification Challenges

• Conventional biochemical methods struggle to differentiate S. salivarius from other viridans species, requiring molecular techniques for precise identification. 2, 3

• MALDI-TOF mass spectrometry provides rapid, reliable species-level identification of S. salivarius and other viridans streptococci with 100% consistency compared to genotypic methods. 4

• The coaE gene sequencing method can precisely differentiate S. salivarius from closely related salivarius group members (S. vestibularis, S. thermophilus) when MALDI-TOF is unavailable. 2

• Automated blood culture systems detect S. salivarius readily, and standard subculture media support growth (unlike nutritionally variant streptococci which require supplemented media). 5, 3

Clinical Significance

Endocarditis Risk

• Both S. salivarius and other viridans streptococci cause 40-60% of community-acquired native valve endocarditis in non-IDU patients, with no clinically meaningful difference in pathogenicity between S. salivarius and other viridans species. 1, 6

• S. salivarius follows the typical viridans pattern: subacute presentation, association with dental procedures, and predilection for previously normal valves. 6, 7

Rare Invasive Infections

• S. salivarius can cause bacteremia and meningitis following gastrointestinal procedures, though this is uncommon and often initially dismissed as contamination. 7

• Unlike the S. anginosus group (S. intermedius, anginosus, constellatus), S. salivarius does NOT have the characteristic abscess-forming tendency that would require prolonged therapy or systematic imaging for metastatic foci. 1

Antimicrobial Therapy

For Highly Penicillin-Susceptible Strains (MIC ≤0.12 μg/mL)

S. salivarius endocarditis is treated identically to other viridans streptococcal endocarditis—there is no species-specific modification required. 1

• Preferred regimen: Penicillin G 12-18 million units/24h IV continuously or in 4-6 divided doses for 4 weeks (Class IA). 1

• Alternative: Ceftriaxone 2g/24h IV/IM once daily for 4 weeks (Class IA). 1

• Short-course option: Penicillin G or ceftriaxone PLUS gentamicin 3 mg/kg/24h for 2 weeks (Class IB), but NOT for patients with abscesses, creatinine clearance <20 mL/min, or 8th nerve dysfunction. 1

• β-lactam allergy: Vancomycin 30 mg/kg/24h IV in 2 divided doses for 4 weeks (Class IB). 1

Resistance Considerations

• Penicillin resistance among viridans streptococci (including S. salivarius) has increased to 13-50% in recent studies, with macrolide resistance reaching 22-58%. 1

• MIC testing is mandatory to guide therapy selection, as treatment regimens are subdivided by penicillin MIC categories (≤0.12,0.12-0.5, >0.5 μg/mL). 1

• Penicillin tolerance (MBC >> MIC) may occur but has no proven clinical significance and should not alter therapy selection. 1

Critical Clinical Pitfalls

• Do not confuse S. salivarius with nutritionally variant streptococci (Abiotrophia, Granulicatella), which require enterococcal-type combination therapy for 6 weeks due to poor cure rates with standard viridans regimens. 5

• Do not extend therapy duration for S. salivarius unless there are complications (abscesses, prosthetic valve)—unlike S. anginosus group infections which may require longer courses. 1

• Blood culture positivity does not automatically indicate pathogenicity—S. salivarius bacteremia may represent transient bacteremia or contamination, requiring clinical correlation. 7

• When S. salivarius is isolated, notify the laboratory if growth fails on subculture, as this would suggest misidentification and possible nutritionally variant streptococci requiring special media. 5