Bactrim DS is NOT Effective Against Group B Streptococcus
Trimethoprim-sulfamethoxazole (Bactrim) should not be used for Group B Streptococcus infections due to inherent resistance and is not recommended for this pathogen. 1
Why Bactrim Fails Against GBS
Universal Resistance Pattern
- All Group B Streptococcus isolates are resistant to sulfonamides (including Bactrim/cotrimoxazole). 2
- Research demonstrates that 100% of GBS strains tested showed resistance to sulphadiazine (a sulfonamide component similar to sulfamethoxazole in Bactrim), with MICs exceeding 500 mg/L. 3
- This resistance is intrinsic to GBS and not acquired, making Bactrim completely ineffective regardless of the clinical scenario. 2
Recommended Treatment Instead
For GBS infections, use penicillin-based antibiotics as first-line therapy:
Perinatal GBS Prevention (Intrapartum Prophylaxis)
- Penicillin G is the preferred agent: 5 million units IV initial dose, then 2.5 million units IV every 4 hours until delivery. 4
- Alternative: Ampicillin 2g IV initial dose, then 1g IV every 4 hours until delivery. 4
- Penicillin is preferred over ampicillin due to its narrower spectrum, which reduces selection pressure for resistant organisms. 4
For Penicillin-Allergic Patients
- Non-anaphylactic allergy: Cefazolin 2g IV initial dose, then 1g IV every 8 hours until delivery. 4
- Anaphylactic/immediate hypersensitivity: Clindamycin 900mg IV every 8 hours OR erythromycin 500mg IV every 6 hours (only if susceptibility testing confirms sensitivity). 4
- If susceptibility unknown or resistant to clindamycin/erythromycin: Vancomycin 1g IV every 12 hours until delivery. 4
Critical Resistance Considerations
- All GBS isolates remain universally susceptible to penicillin, ampicillin, cefazolin, cefotaxime, and vancomycin. 5
- Clindamycin resistance has increased from 10.5% to 15.0%, and erythromycin resistance from 15.8% to 32.8% over surveillance periods. 5
- Approximately 20% of GBS isolates are resistant to clindamycin, requiring susceptibility testing before use. 6
Common Pitfall to Avoid
Never use Bactrim for streptococcal infections of any kind (Group A, B, C, F, or G), as resistance rates are extremely high (50% for Group A Strep, 100% for Group B Strep). 1, 2 This is a fundamental principle in treating streptococcal infections—beta-lactams are the cornerstone of therapy, not sulfonamides.