Is Streptococcus agalactiae (Group B Streptococcus, GBS) considered a culture contaminant in blood cultures?

Streptococcus agalactiae (GBS) is NOT a Blood Culture Contaminant

Streptococcus agalactiae (Group B Streptococcus, GBS) is a true pathogen and should never be dismissed as a contaminant in blood cultures. When isolated from blood cultures, it represents a clinically significant infection requiring prompt treatment.

Pathogenic Nature of GBS

GBS is a gram-positive coccus that causes invasive disease primarily in:

• Newborns (early-onset and late-onset disease)
• Pregnant women
• Adults with underlying medical conditions (especially diabetes mellitus and cancer) 1

Clinical Significance in Different Populations

1. Neonates:

   • Leading cause of neonatal sepsis and meningitis
   • Case-fatality rates of 4-6% in recent years, but up to 20-30% in preterm infants 1
   • Acquired vertically through exposure to GBS from the vagina of a colonized mother

2. Pregnant/Postpartum Women:

   • Causes urinary tract infections, amnionitis, endometritis, and wound infections
   • Associated with stillbirths and premature delivery 1

3. Non-pregnant Adults:

   • Causes skin/soft tissue infections, bacteremia, genitourinary infections, and pneumonia
   • Higher mortality rates (15-32%) than in neonates 1
   • Particularly virulent in elderly and immunocompromised patients

Distinguishing Pathogens from Contaminants

The CDC guidelines for prevention of intravascular catheter-related infections clearly distinguish between true pathogens and contaminants in blood cultures 1:

• True Pathogens: Organisms like GBS are recognized pathogens when cultured from blood and are not considered contaminants
• Contaminants: Common skin contaminants include diphtheroids, Bacillus spp., Propionibacterium spp., coagulase-negative staphylococci, or micrococci

GBS is specifically listed as a recognized pathogen, not as a potential contaminant, in the CDC laboratory-confirmed BSI criteria 1.

Clinical Implications

When GBS is isolated from blood cultures:

• Immediate action required: Start appropriate antimicrobial therapy
• Investigation needed: Identify the source of infection (e.g., skin/soft tissue, urinary tract, endocarditis)
• No repeat cultures necessary: Unlike with potential contaminants, a single positive blood culture with GBS is clinically significant

High-Risk Conditions Associated with GBS Bacteremia

• Diabetes mellitus
• Advanced age
• Malignancy
• Immunosuppression
• Skin breakdown or ulcers
• Recent surgery

Potential Complications of GBS Bacteremia

If left untreated, GBS bacteremia can lead to severe complications including:

• Infective endocarditis 2
• Streptococcal toxic shock-like syndrome 3
• Necrotizing fasciitis
• Meningitis
• Septic arthritis
• Osteomyelitis

Treatment Considerations

For confirmed GBS bacteremia:

• First-line treatment: Intravenous penicillin G
• Alternative: Ampicillin
• For penicillin-allergic patients: Clindamycin (if susceptible) or vancomycin
• Duration typically 10-14 days depending on source and complications

Conclusion

GBS is a significant pathogen with substantial morbidity and mortality. When isolated from blood cultures, it represents true infection requiring prompt and appropriate antimicrobial therapy. Unlike common skin contaminants that may require multiple positive cultures to confirm significance, a single positive blood culture for GBS should be considered clinically relevant and treated accordingly.