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

Group B Streptococcus in Urine and Susceptibility to Macrobid (Nitrofurantoin)

Yes, Group B Streptococcus (GBS) in urine is generally susceptible to Macrobid (nitrofurantoin), which is recommended by the World Health Organization as a first-choice option for GBS urinary tract infections. 1

Susceptibility Profile of GBS

GBS isolates from urinary tract infections typically demonstrate the following antibiotic susceptibility patterns:

• Highly susceptible to:

  • Penicillin G and ampicillin (no resistance documented) 2
  • Cephalosporins (cefazolin, cefuroxime, cefoperazone) 2
  • Nitrofurantoin (Macrobid) 1
  • Vancomycin 3

• Variable susceptibility to:

  • Erythromycin (resistance rates 8-44.5%) 2, 4
  • Clindamycin (resistance rates 26-28%) 2, 4
  • Azithromycin (resistance rate 44.5%) 4
  • Tetracycline (resistance rate 81.5%) 4

Treatment Recommendations for GBS in Urine

For Non-Pregnant Adults:

Nitrofurantoin (Macrobid) is an appropriate choice for treating GBS urinary tract infections in non-pregnant adults, as it is included among the WHO's first-choice options 1. All GBS isolates have been found to be susceptible to beta-lactam antibiotics, making penicillins and cephalosporins also effective options 2, 5.

For Pregnant Women:

Special considerations apply for pregnant women with GBS bacteriuria:

1. Any GBS bacteriuria during pregnancy (regardless of colony count) requires:

   • Treatment at the time of diagnosis 6
   • Intrapartum antibiotic prophylaxis during labor or rupture of membranes 7, 1, 6

2. First-line treatment options for GBS bacteriuria in pregnancy include:

   • Penicillin G
   • Ampicillin
   • Amoxicillin-clavulanic acid
   • Nitrofurantoin
   • Sulfamethoxazole-trimethoprim (avoid in late pregnancy) 1

Clinical Considerations

• GBS in urine at any concentration during pregnancy is a marker for heavy genital tract colonization 7
• Women with GBS bacteriuria during pregnancy should not be re-screened by genital tract culture or urinary culture in the third trimester 6
• GBS urinary tract infections complicate 2-4% of pregnancies 7
• In non-pregnant adults, GBS accounts for approximately 2% of positive urine cultures 5
• GBS UTIs in non-pregnant adults are often associated with underlying urinary tract abnormalities (60%) or chronic renal failure (27%) 5

Antibiotic Resistance Considerations

While GBS remains universally susceptible to beta-lactams and nitrofurantoin, there is increasing resistance to macrolides and lincosamides:

• Erythromycin resistance has increased from 8% in 1998 to 16-36.3% in recent years 2, 4
• Clindamycin resistance has increased from 20% to 26-28% 2, 4
• Multidrug resistance (MDR) has been reported in 33.6% of GBS UTI isolates 4

Pitfalls to Avoid

1. Do not use macrolides (erythromycin, azithromycin) as empiric therapy for GBS UTIs due to increasing resistance rates 2
2. Do not assume that treatment of GBS bacteriuria in pregnancy eliminates the need for intrapartum prophylaxis 7, 6
3. Do not re-screen pregnant women with documented GBS bacteriuria in the third trimester 6
4. Consider underlying urinary tract abnormalities in non-pregnant adults with GBS UTIs 5

In summary, nitrofurantoin (Macrobid) is an effective treatment option for GBS urinary tract infections, with the WHO recommending it as a first-choice therapy. This is particularly important given the increasing resistance of GBS to macrolides and other antibiotic classes.