What are the best antibiotics for treating a urine culture positive for Beta-hemolytic Streptococcus (BHS), group B?

Treatment of Group B Streptococcus (GBS) Urinary Tract Infection

For a urine culture positive for Group B Streptococcus, ampicillin 500 mg orally every 8 hours for 3-7 days is the first-line treatment for uncomplicated UTIs in adults. 1

First-Line Antibiotic Options

Penicillin-based antibiotics remain the gold standard for GBS infections due to universal susceptibility and narrow spectrum activity. 2

• Ampicillin 500 mg orally every 8 hours for 3-7 days is recommended as first-line therapy for uncomplicated GBS UTIs 1
• Amoxicillin 500 mg orally every 8 hours can be used as an alternative with similar efficacy 1
• For severe or complicated infections requiring IV therapy, ampicillin 2 g IV initially, then 1 g IV every 4-6 hours is appropriate 2, 3
• Penicillin G 5 million units IV initially, then 2.5 million units IV every 4 hours is preferred for severe infections due to its narrower spectrum 2, 4

Treatment Duration Based on Infection Severity

The duration of therapy must be tailored to infection complexity:

• Uncomplicated UTIs: 3-7 days of oral therapy 1
• Complicated UTIs: 5-7 days of therapy 1
• Severe infections or bacteremia: 10-14 days of therapy 1
• Any GBS infection: minimum 10 days to prevent rheumatic fever complications 3, 5

Penicillin-Allergic Patients

For patients with penicillin allergy, the choice of alternative antibiotic depends on the severity of the allergic reaction. 2

Non-Severe Penicillin Allergy (No History of Anaphylaxis)

• Cefazolin 2 g IV initial dose, then 1 g IV every 8 hours until infection resolves 2, 1
• This applies to patients without history of anaphylaxis, angioedema, respiratory distress, or urticaria to penicillins or cephalosporins 2

Severe Penicillin Allergy (High Risk for Anaphylaxis)

Antimicrobial susceptibility testing must be performed before selecting clindamycin or erythromycin due to increasing resistance patterns. 2, 6

• Clindamycin 900 mg IV every 8 hours or 300-450 mg orally every 6 hours if the isolate is confirmed susceptible 2, 1, 5
• Vancomycin 1 g IV every 12 hours may be used for severe infections or when susceptibility testing shows resistance to clindamycin/erythromycin 2, 1
• Erythromycin 500 mg IV every 6 hours if susceptibility is confirmed 2

Critical Caveat on Resistance

Resistance to clindamycin and erythromycin among GBS isolates is increasing significantly. 6

• Studies show 23.1% inducible resistance to erythromycin and 10.7% to clindamycin 6
• Constitutive resistance was detected in 14.9% of cases 6
• Always obtain susceptibility testing for penicillin-allergic patients at high risk for anaphylaxis 2
• If susceptibility results are unavailable or show resistance, vancomycin is the appropriate choice 2, 1

Special Considerations for Pregnancy

Pregnant women with GBS bacteriuria require treatment and are automatically candidates for intrapartum antibiotic prophylaxis regardless of later screening results. 2

• Fluoroquinolones must be avoided in pregnancy 1
• Penicillin G, ampicillin, or cefazolin remain the preferred agents 1
• GBS bacteriuria in pregnancy indicates heavy colonization and requires both treatment of the UTI and intrapartum prophylaxis at delivery 2
• For intrapartum prophylaxis: penicillin G 5 million units IV initially, then 2.5 million units IV every 4 hours until delivery 2

Important Clinical Considerations

Obtain urine culture before initiating therapy to confirm diagnosis and guide treatment. 1

• Significant bacteriuria is defined as ≥50,000 CFUs/mL of a single urinary pathogen 1
• Distinguish between colonization and true infection to avoid unnecessary treatment 1
• For complicated or recurrent infections, evaluate for structural urinary tract abnormalities 1
• Consider follow-up urine culture after treatment completion to ensure eradication, especially in complicated cases 1

Key Pitfalls to Avoid

Do not use oral antimicrobial agents to treat GBS colonization detected on prenatal screening - this is ineffective in eliminating carriage or preventing neonatal disease. 2

• All GBS isolates remain universally sensitive to beta-lactam antibiotics 6, 7
• However, some isolates show intermediate sensitivity requiring higher doses 7
• Never assume susceptibility to clindamycin or erythromycin without testing 2, 6
• For patients with GBS bacteriuria, nitrofurantoin is an appropriate alternative based on sensitivity patterns 7