I have a patient with a urine culture growing Streptococcus agalactiae (group B beta‑hemolytic streptococcus) at 25,000–50,000 CFU/mL; I ordered trimethoprim‑sulfamethoxazole (Bactrim). What is the appropriate antibiotic therapy?

Inappropriate Antibiotic Choice: Switch from Bactrim to Beta-Lactam Therapy

Trimethoprim-sulfamethoxazole (Bactrim) is not an appropriate treatment for Group B Streptococcus (GBS) urinary tract infection and should be changed immediately to a beta-lactam antibiotic such as amoxicillin, ampicillin, or a first-generation cephalosporin. 1, 2

Why Bactrim is Inappropriate

• GBS is intrinsically resistant to trimethoprim-sulfamethoxazole, making this antibiotic choice ineffective regardless of susceptibility testing 3
• The CLSI guidance you cited specifically refers to beta-lactam antibiotics (penicillins and cephalosporins) as drugs of choice, not sulfonamides 1
• TMP-SMX has no reliable activity against streptococcal species and should never be used for streptococcal infections 4

Recommended Treatment Options

First-Line Therapy (Beta-Lactams)

• Amoxicillin 500 mg orally three times daily is the preferred oral agent for uncomplicated GBS UTI 1, 2
• Ampicillin (if parenteral therapy needed initially) followed by oral step-down 1, 2
• First-generation cephalosporins (cephalexin 500 mg four times daily) are acceptable alternatives 2

Key Supporting Evidence

• All GBS isolates remain universally susceptible to penicillins and ampicillin with no documented resistance 1, 2
• Beta-lactam antibiotics achieve excellent urinary concentrations and are highly effective for GBS bacteriuria 2, 4
• Research confirms 100% sensitivity of GBS clinical isolates to penicillin and ampicillin 1, 2

Clinical Significance of This Colony Count

• 25,000-50,000 CFU/mL represents significant bacteriuria requiring treatment, particularly with a pathogen like GBS 3
• This colony count exceeds the traditional threshold of ≥10,000 CFU/mL for uropathogens in symptomatic patients 3
• GBS bacteriuria can lead to serious complications including bacteremia, particularly in patients with diabetes, immunosuppression, or urologic abnormalities 5

Treatment Duration

• 7-10 days of oral beta-lactam therapy is recommended for uncomplicated GBS UTI 3
• Shorter 3-day courses are not appropriate for GBS infections, unlike E. coli cystitis 3

Important Caveats

• Do not use macrolides or clindamycin for GBS UTI despite their effectiveness in other GBS infections, as resistance rates are increasing (14.9-23.1% constitutive and inducible resistance reported) 1
• Nitrofurantoin shows excellent activity against GBS (98% susceptibility) and could be considered as an alternative if beta-lactam allergy exists, though beta-lactams remain preferred 2
• If true penicillin allergy is documented, obtain formal allergy history as most patients can safely receive cephalosporins 2

Action Items

1. Discontinue Bactrim immediately 1, 2
2. Start amoxicillin 500 mg orally three times daily for 7-10 days 1, 2
3. No repeat susceptibility testing is needed for beta-lactams as resistance is essentially nonexistent 1
4. Consider repeat urine culture only if symptoms persist after 48-72 hours of appropriate therapy 3