What is the recommended dosing of Bactrim (sulfamethoxazole/trimethoprim) for an E. coli infection?

Bactrim Dosing for E. coli Infection

For E. coli infections, the recommended dosage of Bactrim (sulfamethoxazole/trimethoprim) is 160/800 mg (one double-strength tablet) twice daily for 10-14 days for urinary tract infections in adults. 1

Adult Dosing Recommendations

Urinary Tract Infections

• Standard dose: 160/800 mg (one double-strength tablet) twice daily for 10-14 days 1
• For uncomplicated cystitis, treatment may be effective with a shorter duration (3-7 days) based on clinical response

E. coli Diarrheal Infections

• Traveler's diarrhea: 160/800 mg twice daily for 3-5 days 2, 1
• Shigellosis: 160/800 mg twice daily for 5 days 1

Dosing in Renal Impairment

• Creatinine clearance >30 mL/min: Standard regimen
• Creatinine clearance 15-30 mL/min: Half the usual regimen
• Creatinine clearance <15 mL/min: Not recommended 1

Pediatric Dosing Recommendations

Children 2 months and older

• Dose: 8 mg/kg trimethoprim and 40 mg/kg sulfamethoxazole per 24 hours, divided into two doses given every 12 hours 1
• Duration: 10 days for UTI; 5 days for shigellosis

Weight-based dosing guide

• 10 kg (22 lb): 5 mL (1 teaspoon) every 12 hours
• 20 kg (44 lb): 10 mL (2 teaspoons) every 12 hours
• 30 kg (66 lb): 15 mL (3 teaspoons) every 12 hours
• 40 kg (88 lb): 20 mL (4 teaspoons) every 12 hours 1

Important Considerations

Resistance Patterns

• Local antimicrobial resistance patterns should guide therapy 2
• TMP-SMX should be avoided if local E. coli resistance exceeds 20% 3
• Risk factors for TMP-SMX resistance include:
  • Recurrent UTIs
  • Genitourinary abnormalities
  • TMP-SMX use within the past 90 days 3
  • Multiple UTI episodes in the past year 4

Alternative Therapies

If TMP-SMX resistance is suspected or confirmed:

• Fluoroquinolones: Ciprofloxacin 500 mg twice daily for 7 days
• Cephalosporins: Various options based on specific agent
• Nitrofurantoin: For uncomplicated lower UTIs only (not for pyelonephritis) 2

Monitoring

• Clinical response should be evident within 48-72 hours
• Consider follow-up urine culture in complicated cases or treatment failures
• Monitor for adverse effects including rash, GI symptoms, and rare but serious reactions

Pitfalls to Avoid

1. Not considering local resistance patterns: Always consider local E. coli resistance to TMP-SMX before prescribing
2. Using inadequate duration: Too short a course may lead to treatment failure
3. Using in severe renal impairment: Avoid in patients with creatinine clearance <15 mL/min
4. Not adjusting for patient-specific factors: Consider age, comorbidities, and risk factors for resistance
5. Ignoring allergies: Approximately 3% of patients may have allergic reactions to sulfamethoxazole, some of which can be serious 5

TMP-SMX remains an effective first-line option for E. coli infections when local resistance rates are acceptable and patient-specific risk factors for resistance are absent.