What is the recommended treatment for a 6-month-old patient with Group B Streptococcus (GBS) in the urine?

Treatment of GBS in Urine for a 6-Month-Old

For a 6-month-old with GBS bacteriuria, initiate intravenous ampicillin 200 mg/kg/day divided every 6 hours plus gentamicin (an aminoglycoside) as empiric therapy, pending culture confirmation and clinical assessment. 1, 2

Initial Empiric Antibiotic Regimen

The European Association of Urology and European Society for Paediatric Urology guidelines specifically recommend for urinary tract infections in infants aged 6 months or younger:

• Parenteral ampicillin PLUS an aminoglycoside (typically gentamicin) 1
• Alternative regimen: Ceftazidime plus ampicillin 1

Ampicillin dosing: 200 mg/kg/day administered intravenously divided every 6 hours 2

Gentamicin dosing: Weight and gestational age-based dosing per institutional protocols (typically 4-5 mg/kg/dose every 24-48 hours depending on postnatal age) 3, 4

Why Combination Therapy is Critical at This Age

• Dual coverage is essential because urinary tract infections in infants under 6 months require coverage for both GBS and gram-negative organisms (especially E. coli and other Enterobacteriaceae) until culture results confirm the pathogen 1, 4
• The combination of a penicillin-derivative (ampicillin) with an aminoglycoside provides synergistic bactericidal activity and is the standard of care for serious infections in this age group 2, 4
• Do not use monotherapy initially in infants this young with UTI, as the risk of serious bacterial infection including sepsis is substantial 1, 4

Once GBS is Confirmed

After culture confirmation of GBS and clinical improvement:

• Narrow to ampicillin monotherapy at 200 mg/kg/day IV divided every 6 hours 2
• Alternative: Penicillin G at 100,000-250,000 units/kg/day IV divided every 4-6 hours 2
• Discontinue the aminoglycoside once GBS is confirmed and the infant is clinically improving, to minimize nephrotoxicity and ototoxicity risk 2, 4

Duration of Therapy

• Uncomplicated GBS UTI: 10-14 days of therapy 2, 4
• If bacteremia is present: Minimum 10-14 days 2
• If clinical concern for pyelonephritis or systemic involvement: Consider extended duration and obtain blood cultures 1, 2

Step-Down to Oral Therapy

Once the infant shows clear clinical improvement (typically after 48-72 hours of IV therapy and afebrile for 24 hours):

• Amoxicillin 50-75 mg/kg/day divided into 2 doses orally to complete the treatment course 2
• Alternative: Penicillin V 50-75 mg/kg/day divided into 3-4 doses orally 2

Critical Pitfalls to Avoid

• Never use nitrofurantoin, fluoroquinolones, sulfonamides, or tetracyclines for GBS infections in infants—these lack proven efficacy and are contraindicated in this age group 5
• Do not underdose or prematurely switch to oral therapy before documented clinical improvement, as this leads to treatment failure and recurrence 5, 2
• Avoid monotherapy with ampicillin alone initially in a 6-month-old with UTI until GBS is confirmed, as empiric coverage must include gram-negative organisms 1, 4
• Do not use third-generation cephalosporins as monotherapy for confirmed GBS—penicillin/ampicillin remains superior due to narrow spectrum and proven efficacy 5

Additional Diagnostic Considerations

• Obtain blood cultures in addition to urine culture, as GBS bacteriuria in infants can indicate systemic infection 2, 4
• Consider renal ultrasound to evaluate for structural abnormalities, as UTI in infants this young warrants imaging 1
• Monitor closely for 48-72 hours for signs of clinical deterioration or sepsis 1, 2

Penicillin Allergy Considerations

If the infant has a documented severe penicillin allergy:

• Cefazolin 25-50 mg/kg/dose IV every 8 hours (for non-anaphylactic allergy) 5, 2
• Clindamycin 40 mg/kg/day IV divided every 6-8 hours ONLY if GBS susceptibility is confirmed (approximately 20% of GBS isolates are clindamycin-resistant) 2
• Vancomycin may be considered for severe beta-lactam allergies, though this is rarely necessary for GBS UTI 5