Is Streptococcus agalactiae (Group B) in urine susceptible to Macrobid (nitrofurantoin)?

Susceptibility of Streptococcus agalactiae (Group B) to Nitrofurantoin

Nitrofurantoin is an effective treatment option for urinary tract infections caused by Streptococcus agalactiae (Group B Streptococcus or GBS), with research showing very low resistance rates (0%) to this antibiotic. 1

Antimicrobial Susceptibility Profile of GBS

GBS isolates generally demonstrate the following susceptibility patterns:

• Highly susceptible to:

  • Nitrofurantoin (0% resistance) 1
  • Penicillin (4.2% resistance) 1
  • Ampicillin (2.7% resistance) 1
  • Vancomycin (2.4% resistance) 1
  • Cefazolin (7.6% resistance) 1

• Moderate to high resistance to:

  • Tetracycline (96.2% resistance) 1
  • Trimethoprim-sulfamethoxazole (84.7% resistance) 1
  • Cefotaxime (41.3% resistance) 1
  • Clindamycin (26.8% resistance) 1
  • Erythromycin (21% resistance) 1

Treatment Recommendations for GBS UTIs

The World Health Organization (WHO) recommends nitrofurantoin as one of the first-choice options for the treatment of lower urinary tract infections 2. This recommendation is supported by data showing that susceptibility of E. coli and other urinary pathogens to nitrofurantoin remains generally high.

For GBS specifically:

1. First-line options:

   • Penicillin G or ampicillin (particularly in pregnant women) 3
   • Nitrofurantoin for uncomplicated lower UTIs 3

2. Alternative options for penicillin-allergic patients:

   • Cefazolin (if no anaphylactic reaction history) 3
   • Clindamycin (only if susceptibility confirmed) 3
   • Vancomycin (for clindamycin-resistant strains) 3

Special Considerations for GBS in Urine

Pregnancy

If GBS is detected in urine during pregnancy:

• Immediate treatment of the current infection is required
• Intrapartum antibiotic prophylaxis during labor is recommended regardless of colony count 2, 3
• GBS in urine is considered a marker for heavy genital tract colonization 2

Laboratory Reporting

• Laboratories should report GBS in urine culture specimens when present at concentrations of ≥10^4 colony-forming units/ml in pure culture or mixed with a second microorganism 2

Historical Perspective on Nitrofurantoin and GBS

Early research from 1982 showed that the minimum inhibitory concentrations (MICs) for nitrofurantoin against GBS ranged from 8 to >64 mg/l 4. However, more recent studies indicate improved susceptibility patterns, with current meta-analyses showing 0% resistance to nitrofurantoin among GBS isolates 1.

Clinical Application

When treating a urinary tract infection with GBS:

1. For uncomplicated lower UTIs:

   • Nitrofurantoin is an effective option with minimal resistance
   • Standard dosing: 100 mg orally twice daily for 5-7 days

2. For complicated UTIs or pyelonephritis:

   • Beta-lactams (penicillin, ampicillin) are preferred
   • Treatment duration: 10-14 days 3

3. For pregnant patients:

   • Any GBS in urine requires treatment regardless of colony count
   • Follow-up urine culture after treatment completion is recommended 3
   • Intrapartum prophylaxis will be needed during labor 2, 3

Conclusion

Nitrofurantoin remains a highly effective option for treating GBS urinary tract infections with virtually no resistance reported in recent studies. Its high urinary concentrations and minimal toxicity make it particularly suitable for uncomplicated lower UTIs caused by this organism.