What are the recommended antibiotics for a patient with Group B strep (Streptococcus agalactiae) bacteremia?

Treatment of Group B Streptococcus Bacteremia

For adults with Group B Streptococcus bacteremia, penicillin G 12-24 million units/day IV in divided doses (2-4 million units every 4 hours) is the first-line treatment, with ampicillin 2 g IV every 4-6 hours as an acceptable alternative. 1, 2

First-Line Antibiotic Regimens

• Penicillin G remains the preferred agent due to its narrow spectrum of activity, universal GBS susceptibility, and proven efficacy in serious streptococcal infections including bacteremia 1, 2
• The FDA-approved dosing for serious streptococcal infections is 12-20 million units/day divided in equal doses, though some sources recommend up to 24 million units/day for severe bacteremia 2
• Ampicillin 2 g IV initially, then 1-2 g IV every 4-6 hours is an acceptable alternative, though it has broader spectrum activity which may promote resistance 3, 1

Treatment Duration and Monitoring

• Continue IV antibiotics for at least 10-14 days for uncomplicated bacteremia, though duration should be extended based on clinical response and source control 3, 2
• Treatment should continue for at least 48-72 hours after the patient becomes asymptomatic, per FDA labeling 2
• If endocarditis is present, extend treatment to 4-6 weeks with penicillin G 12-20 million units/day 2
• If meningitis is documented, use penicillin G 12-24 million units/day (as 2-4 million units every 4 hours) for 10-14 days 2

Penicillin-Allergic Patients

• For non-severe penicillin allergy (no history of anaphylaxis, angioedema, or urticaria): Use cefazolin 2 g IV initially, then 1 g IV every 8 hours 3, 1
• For severe penicillin allergy with high anaphylaxis risk: Use clindamycin 900 mg IV every 8 hours IF the isolate is confirmed susceptible to both clindamycin and erythromycin 3, 1
• If clindamycin susceptibility is unknown or the isolate is resistant: Use vancomycin 1 g IV every 12 hours 3, 1
• Susceptibility testing is mandatory for penicillin-allergic patients, as clindamycin resistance rates have increased to 3-15% in the US and erythromycin resistance reaches up to 20% 1, 4

Critical Clinical Considerations

• All GBS isolates remain universally susceptible to penicillin G, ampicillin, cephalosporins, and vancomycin 4
• The risk of penicillin anaphylaxis (4-5 per 10,000 to 100,000 recipients) is far outweighed by the benefits of appropriate treatment 1
• Macrolides (erythromycin) should never be used empirically due to rising resistance rates and documented treatment failures 1, 4
• Intravenous administration is mandatory for bacteremia—oral antibiotics are inadequate for bloodstream infections 1

High-Risk Populations and Prognosis

• GBS bacteremia predominantly affects elderly patients (mean age 63 years) with severe underlying conditions including liver disease (35%), malignancies (33%), and diabetes (28%) 4
• Overall mortality for GBS bacteremia is 25-33%, with deaths directly attributable to GBS in approximately 25% of cases 4
• Poor prognostic factors include: central nervous system disease, alcoholism, shock, renal failure, and altered consciousness 4
• The incidence of GBS bacteremia has significantly increased over time (from 0.08 to 0.3 per 1000 admissions) 4

Renal Dosing Adjustments

• For creatinine clearance <10 mL/min/1.73m²: Give full loading dose, then half the loading dose every 8-10 hours 2
• For creatinine clearance >10 mL/min/1.73m²: Give full loading dose, then half the loading dose every 4-5 hours 2
• Additional modifications are needed for combined hepatic and renal impairment 2

Common Pitfalls to Avoid

• Never rely on oral antibiotics for bacteremia treatment—they achieve inadequate serum concentrations and lead to treatment failure 1, 5
• Do not use underdosing or premature discontinuation, as this leads to treatment failure or recurrence 3
• Avoid empiric clindamycin without susceptibility testing due to rising resistance rates 1, 4
• Do not assume penicillin allergy without verification—many reported allergies are not true IgE-mediated reactions 6