Antibiotics Effective Against Group B Streptococcus (GBS)
Penicillin G remains the first-line antibiotic for Group B Streptococcus infections due to its high efficacy and safety profile. 1, 2
First-Line Antibiotics for GBS
- Penicillin G: 5 million units IV initial dose, then 2.5-3.0 million units IV every 4 hours until delivery/resolution 1
- Ampicillin: 2 g IV initial dose, then 1 g IV every 4 hours 1
- Cefazolin: 2 g IV initial dose, then 1 g IV every 8 hours (for patients with non-anaphylactic penicillin allergy) 1
Alternative Antibiotics for Penicillin-Allergic Patients
For patients with severe penicillin allergy (history of anaphylaxis, angioedema, respiratory distress, or urticaria):
- Clindamycin: 600 mg IV every 8 hours (only if GBS isolate is confirmed susceptible to both clindamycin and erythromycin, and negative for inducible clindamycin resistance) 3, 1
- Vancomycin: 15-20 mg/kg IV every 12 hours (if GBS susceptibility to clindamycin is unknown or shows resistance) 3, 1, 4
Antimicrobial Susceptibility Considerations
- All GBS isolates are generally susceptible to penicillin, ampicillin, and vancomycin 2, 4, 5
- Resistance to macrolides (erythromycin, azithromycin) and clindamycin is increasing, with studies showing resistance rates of up to 31% for azithromycin and 19% for clindamycin 5
- For penicillin-allergic patients, antimicrobial susceptibility testing should be performed on GBS isolates to guide therapy 3, 1
Treatment Duration
- For uncomplicated GBS infections: 7-10 days of therapy 1
- For complicated infections (slow clinical response, undrainable foci, immunodeficiency): 14 days or longer 1
- For GBS bacteremia: continue treatment until blood cultures are negative and clinical improvement is observed 1
Special Considerations
- Penicillin G achieves adequate drug levels in fetal circulation and amniotic fluid while avoiding neurotoxicity 3
- For urinary tract infections with GBS, nitrofurantoin may be effective as studies show minimal resistance 5
- In pregnant women at risk for rapid delivery, intramuscular benzathine penicillin G (Bicillin L-A) may be used as adjunctive therapy, though it's insufficient as sole therapy 6
Monitoring and Follow-up
- For GBS bacteremia, obtain follow-up blood cultures to document clearance 1
- Consider oral step-down therapy after clinical improvement, bacteremia clearance, and exclusion of endocarditis or metastatic abscess 1
Key Pitfalls to Avoid
- Failing to perform susceptibility testing in penicillin-allergic patients, which can lead to treatment failure
- Using erythromycin for GBS prophylaxis, which is no longer recommended due to increasing resistance 3, 5
- Inadequate dosing intervals - penicillin G should be administered every 4 hours to maintain effective concentrations 7
- Not considering the severity of penicillin allergy when selecting alternative antibiotics
- Treating asymptomatic GBS colonization outside of specific situations like pregnancy 1
By following these evidence-based recommendations, clinicians can effectively treat Group B Streptococcus infections while minimizing the risk of treatment failure and antibiotic resistance.