What is the first‑line treatment for an uncomplicated Group B Streptococcus urinary tract infection in a non‑pregnant adult without a penicillin allergy?

Treatment of Group B Streptococcus UTI in Non-Pregnant Adults

For uncomplicated Group B Streptococcus urinary tract infection in a non-pregnant adult without penicillin allergy, penicillin or ampicillin is the first-line treatment, as all GBS isolates worldwide remain 100% susceptible to these agents. 1

First-Line Antibiotic Regimens

Oral therapy options for outpatient management:

• Amoxicillin 500 mg three times daily for 3-7 days is appropriate for uncomplicated lower UTI 2, 3
• Ampicillin 500 mg four times daily for 3-7 days is an alternative 3

Intravenous therapy for severe or complicated cases:

• Penicillin G 5 million units IV initially, then 2.5-3.0 million units IV every 4 hours 1
• Ampicillin 2g IV initially, then 1g IV every 4 hours 1

Clinical Context and Pathogen Significance

GBS accounts for approximately 2% of positive urine cultures in non-pregnant adults and signals the need for screening for underlying urinary tract abnormalities 4. In one prospective study, 85% of non-pregnant adults with GBS UTI were women, and 95% had at least one underlying condition, most commonly urinary tract abnormalities (60%) and chronic renal failure (27%) 4.

Clinical manifestations are equally distributed between upper and lower urinary tract infections (37% versus 38%), and despite treatment, clinical outcomes were poor in 18% of episodes 4. This underscores the importance of appropriate antibiotic selection and consideration of underlying structural abnormalities.

Treatment Duration

For uncomplicated cystitis, a 3-day course is generally adequate 5, 3. However, if upper tract involvement (pyelonephritis) is suspected based on fever, flank pain, or systemic symptoms, extend treatment to 7-14 days 3.

Alternative Regimens for Penicillin Allergy

For patients with non-severe penicillin allergy (no history of anaphylaxis, angioedema, or urticaria):

• Cefazolin 2g IV initially, then 1g IV every 8 hours 1
• First-generation cephalosporins carry approximately 10% cross-reactivity risk in truly penicillin-allergic patients 6

For patients with severe penicillin allergy:

• Always obtain clindamycin and erythromycin susceptibility testing, as clindamycin resistance ranges from 3-15% among GBS isolates 1
• If susceptible to both clindamycin and erythromycin: Clindamycin 900 mg IV every 8 hours 1
• If resistant to either antibiotic or susceptibility unknown: Vancomycin 1g IV every 12 hours 1
• Perform D-zone testing on isolates that are erythromycin-resistant but clindamycin-susceptible to detect inducible clindamycin resistance 1

Critical Pitfalls to Avoid

Do not use gentamicin or aminoglycosides for GBS UTI, as all isolates in published series were resistant to gentamicin 4. This is a common error when empirically treating gram-positive cocci in urine.

Screen for underlying urinary tract abnormalities when GBS is isolated from urine in non-pregnant adults, as 60% have structural abnormalities that predispose to infection 4. Consider ordering renal ultrasound or CT urography if recurrent infections occur.

Distinguish pregnancy status immediately, as management differs dramatically. Any concentration of GBS in urine during pregnancy requires both immediate treatment and mandatory intrapartum IV prophylaxis during labor, regardless of whether the UTI is treated 7. This question specifically addresses non-pregnant adults, where such prophylaxis is not indicated.

When to Consider Broader Workup

Given that 95% of non-pregnant adults with GBS UTI have underlying conditions 4, evaluate for:

• Diabetes mellitus
• Chronic kidney disease (27% prevalence in GBS UTI patients) 4
• Structural urinary tract abnormalities (60% prevalence) 4
• Immunosuppression

The presence of GBS bacteriuria in a non-pregnant adult should prompt investigation for these underlying conditions rather than being dismissed as simple cystitis.