What is the best antibiotic for a urinary tract infection (UTI) caused by beta-hemolytic group B streptococcus (GBS)?

Treatment of Beta-Hemolytic Group B Streptococcus UTI

For Group B Streptococcus (GBS) urinary tract infections, penicillin G or ampicillin are the antibiotics of choice due to universal GBS susceptibility to penicillins, with penicillin G preferred for its narrower spectrum and proven efficacy. 1, 2, 3

First-Line Antibiotic Selection

For Non-Pregnant Patients

• Penicillin G is the preferred agent due to its narrow spectrum of activity and universal GBS susceptibility, though specific oral formulations may be limited in outpatient settings 1, 2
• Ampicillin is an acceptable alternative with proven efficacy against GBS, though it has broader spectrum activity which may increase selection pressure for resistant organisms 1, 3
• All GBS isolates demonstrate consistent sensitivity to penicillin G, ampicillin, and cephalosporins, making these reliable first-line choices 2

For Pregnant Patients - Critical Distinction

Any concentration of GBS in urine during pregnancy requires immediate treatment AND mandatory intrapartum IV antibiotic prophylaxis during labor, regardless of whether the UTI is treated during pregnancy. 1

Acute UTI Treatment During Pregnancy

• Treat the symptomatic UTI immediately using standard pregnancy-safe antibiotics based on susceptibility testing 1
• Important caveat: Treating the UTI does NOT eliminate GBS colonization from the genitourinary tract—recolonization after oral antibiotics is typical 1

Mandatory Intrapartum Prophylaxis Regimens

• Penicillin G 5 million units IV initially, then 2.5-3.0 million units IV every 4 hours until delivery (preferred regimen) 1
• Ampicillin 2 g IV initially, then 1 g IV every 4 hours until delivery (acceptable alternative) 1
• Prophylaxis must be administered ≥4 hours before delivery for maximum effectiveness (78% reduction in early-onset neonatal GBS disease) 1

Penicillin-Allergic Patients

Risk Stratification Required

• High-risk for anaphylaxis includes history of anaphylaxis, angioedema, respiratory distress, or urticaria after penicillin or cephalosporin exposure 4, 1
• Approximately 10% of penicillin-allergic patients also react to cephalosporins 1

Non-Pregnant Patients with Penicillin Allergy

• Cephalexin or other first-generation cephalosporins for patients without high-risk allergy symptoms 4
• Clindamycin if susceptible (note: 3-15% of GBS isolates are clindamycin-resistant) 1, 2
• Erythromycin resistance is 7-21% among GBS isolates, making susceptibility testing essential 1

Pregnant Patients with Penicillin Allergy - Intrapartum Regimens

• For non-high-risk allergy: Cefazolin 2 g IV initially, then 1 g IV every 8 hours until delivery 4, 1
• For high-risk allergy with susceptible isolate: Clindamycin 900 mg IV every 8 hours until delivery (requires susceptibility confirmation) 4, 1
• For high-risk allergy with resistant or unknown susceptibility: Vancomycin 1 g IV every 12 hours until delivery 4, 1

Susceptibility Testing Requirements

• All penicillin-allergic pregnant patients at high risk for anaphylaxis must have clindamycin and erythromycin susceptibility testing ordered 4, 1
• D-zone testing should be performed on isolates that are erythromycin-resistant but clindamycin-susceptible to detect inducible clindamycin resistance 4, 1
• GBS isolates demonstrate universal penicillin susceptibility, though minimal inhibitory concentrations are higher than for Group A streptococci 2

Alternative Oral Agents for Uncomplicated Cystitis

While penicillins remain first-line, other options for uncomplicated lower UTI include:

• Nitrofurantoin 100 mg PO every 6 hours for uncomplicated UTI 4
• Fosfomycin 3 g PO single dose for uncomplicated UTI 4
• These agents are particularly useful when penicillin allergy exists or for step-down therapy 4

Treatment Duration

• Treat acute cystitis episodes for no longer than 7 days in non-pregnant patients 4
• Shorter courses (5 days) are appropriate for uncomplicated cystitis with first-line agents 4
• Pregnant patients require completion of UTI treatment PLUS intrapartum prophylaxis during labor 1

Critical Pitfalls to Avoid

• Never treat asymptomatic GBS vaginal colonization with oral or IV antibiotics before labor—this is completely ineffective at eliminating colonization and promotes resistance 1, 5
• Do not assume treating a GBS UTI during pregnancy eliminates the need for intrapartum prophylaxis—this is a dangerous error that significantly increases neonatal disease risk 1
• Do not use trimethoprim-sulfamethoxazole or fluoroquinolones as empiric therapy without susceptibility data, as resistance rates are high in many communities 4, 6
• Avoid tigecycline for GBS bacteremia due to low serum levels despite tissue penetration 4
• GBS in urine should not be dismissed as a contaminant—it represents true infection requiring treatment 7

Special Pregnancy Considerations

• GBS bacteriuria at any concentration during pregnancy indicates heavy genital tract colonization and increases neonatal disease risk 29-fold 1
• Women with GBS bacteriuria at any point during pregnancy should not be re-screened with vaginal-rectal cultures at 35-37 weeks—they automatically qualify for intrapartum prophylaxis 1
• For preterm labor or preterm premature rupture of membranes, ampicillin 2 g IV once, followed by 1 g IV every 6 hours for at least 48 hours provides both latency and GBS prophylaxis 1