What is the best antibiotic for a Group B strep (Streptococcus agalactiae) urinary tract infection (UTI)?

Best Antibiotic for Group B Streptococcus UTI

Penicillin G is the first-line antibiotic of choice for treating Group B Streptococcus (GBS) urinary tract infections, with ampicillin as an acceptable alternative. 1

First-Line Treatment Options

For patients without penicillin allergy, the recommended treatment regimens are:

• Penicillin G: 5 million units IV initial dose, then 2.5-3.0 million units IV every 4 hours until resolution 1
• Ampicillin: 2 g IV initial dose, then 1 g IV every 4 hours until resolution 1

These recommendations are supported by strong evidence showing that GBS remains universally susceptible to penicillin and ampicillin. Recent research from 2024 confirms that all tested GBS strains remain fully sensitive to penicillin and ampicillin 2.

Alternative Options for Penicillin-Allergic Patients

For patients with penicillin allergy:

1. Non-severe allergies (no history of anaphylaxis, angioedema, respiratory distress, or urticaria):

   • Cefazolin: 2 g IV initial dose, then 1 g IV every 8 hours 1

2. Severe allergies with susceptible GBS isolate:

   • Clindamycin: 900 mg IV every 8 hours 1

3. Severe allergies with resistant isolate or unknown susceptibility:

   • Vancomycin: 1 g IV every 12 hours 1

Important Considerations

Antibiotic Resistance Patterns

• GBS remains universally susceptible to penicillin and ampicillin 2, 3, 4, 5
• High resistance rates have been documented for:
  • Clindamycin (up to 77.3%) 2
  • Erythromycin (up to 32.9%) 1
  • Tetracycline (up to 88.5%) 2

This increasing resistance to alternative antibiotics makes susceptibility testing crucial for penicillin-allergic patients 1.

Oral Treatment Options

For less severe cases or outpatient management, the WHO recommends:

• Amoxicillin-clavulanic acid
• Nitrofurantoin
• Sulfamethoxazole-trimethoprim 1

Nitrofurantoin shows particularly good activity against GBS with studies showing 98.3% susceptibility 4 and is specifically recommended for GBS bacteriuria 5.

Risk of Anaphylaxis

The risk of anaphylaxis with penicillin is estimated at approximately 5 cases per 10,000 treatments 1, 6. While this risk is relatively low, it should be considered when selecting antibiotics, especially in patients with a history of allergic reactions.

Treatment Algorithm

1. Assess penicillin allergy status
2. For non-allergic patients:
   • Use penicillin G (first choice) or ampicillin
3. For penicillin-allergic patients:
   • Determine severity of allergy
   • Order susceptibility testing
   • Select appropriate alternative based on allergy severity and susceptibility results
4. Monitor response to treatment:
   • Follow-up urine culture may be necessary to confirm clearance
   • Consider longer treatment duration for complicated infections

Pitfalls to Avoid

• Don't use erythromycin without susceptibility testing due to high resistance rates
• Don't use broad-spectrum antibiotics like carbapenems unless multidrug resistance is confirmed 1
• Don't assume clindamycin will be effective without susceptibility testing, as resistance rates are high 1, 2
• Don't treat asymptomatic GBS bacteriuria during pregnancy with oral antibiotics alone, as 30-70% of treated women remain colonized 1

By following these evidence-based recommendations, clinicians can effectively treat GBS UTIs while minimizing the risk of treatment failure and antibiotic resistance.