Streptococcus agalactiae Susceptibility to Nitrofurantoin (Macrobid)
Streptococcus agalactiae (Group B Streptococcus) is susceptible to nitrofurantoin (Macrobid), making it an effective treatment option for uncomplicated lower urinary tract infections caused by this pathogen.
Susceptibility Evidence
The most recent research evidence strongly supports the susceptibility of Streptococcus agalactiae to nitrofurantoin:
A 2020 meta-analysis specifically examining Group B Streptococcus drug resistance in pregnant women found 0% resistance to nitrofurantoin, indicating excellent susceptibility 1.
Earlier research has demonstrated that while S. agalactiae has variable MIC (minimum inhibitory concentration) values for nitrofurantoin ranging from 8 to >64 mg/l, these concentrations are achievable in urine with standard dosing, making it clinically effective 2.
Clinical Application
Nitrofurantoin is particularly valuable for treating lower urinary tract infections (UTIs) for several reasons:
The WHO Essential Medicines and AWaRe guidelines (2024) recommend nitrofurantoin as one of the first-choice options for lower urinary tract infections due to its continued high efficacy against common uropathogens 3.
Nitrofurantoin maintains excellent activity against many urinary pathogens, including S. agalactiae, with minimal development of resistance over its 60+ years of clinical use 4, 5.
It achieves high concentrations in urine but low serum levels, making it ideal for uncomplicated lower UTIs but unsuitable for systemic infections or pyelonephritis 6.
Important Limitations and Contraindications
Despite its effectiveness against S. agalactiae, nitrofurantoin has several important contraindications:
Renal impairment: Nitrofurantoin is contraindicated in patients with significant renal impairment (creatinine clearance <30 mL/min) due to reduced urinary concentration and increased risk of toxicity 6, 5.
Pregnancy: Contraindicated in the last trimester of pregnancy due to risk of hemolytic anemia in the newborn 5.
Systemic infections: Not appropriate for pyelonephritis, prostatitis, or other systemic infections due to inadequate tissue and blood levels 6.
Alternative Treatment Options
For patients who cannot use nitrofurantoin, alternative options for S. agalactiae UTIs include:
Penicillins: S. agalactiae remains highly susceptible to penicillin (only 4.2% resistance) and ampicillin (2.7% resistance) 1, 7.
Cephalosporins: First-generation cephalosporins like cefazolin show good activity (7.6% resistance) 1.
Vancomycin: For penicillin-allergic patients, vancomycin remains effective (2.4% resistance) 1.
Conclusion for Clinical Practice
When treating a lower UTI caused by S. agalactiae:
Nitrofurantoin is an excellent first-line option in patients with normal renal function who are not in the third trimester of pregnancy.
Standard dosing of 100 mg twice daily for 5-7 days is typically effective for uncomplicated lower UTIs.
For patients with contraindications to nitrofurantoin, beta-lactams (penicillins or cephalosporins) are appropriate alternatives, with vancomycin reserved for those with severe penicillin allergies.
Avoid trimethoprim-sulfamethoxazole due to high resistance rates (84.7%) and tetracycline (96.2% resistance) for S. agalactiae 1.