Bactrim Treatment for Chronic Otitis Externa from MRSA and Proteus vulgaris
For chronic otitis externa caused by MRSA and Proteus vulgaris, prescribe trimethoprim-sulfamethoxazole (Bactrim) 1-2 double-strength tablets (160/800 mg) twice daily for 6-12 weeks, combined with topical antibiotic ear drops.
Dosing Regimen
Oral Bactrim dose: Administer 1-2 double-strength tablets (160 mg trimethoprim/800 mg sulfamethoxazole per tablet) twice daily 1, 2, 3
Duration: Treat for 6-12 weeks based on clinical response, with the most pronounced treatment effect occurring with shorter courses 4
Topical therapy: Combine oral Bactrim with topical antibiotic ear drops (gentamicin or polymyxin B-neomycin-hydrocortisone) for synergistic effect 5
Evidence Supporting This Approach
The recommendation for prolonged therapy stems from a randomized controlled trial demonstrating that 6-12 weeks of high-dose oral trimethoprim-sulfamethoxazole (18 mg/kg twice daily, equivalent to 1-2 DS tablets in adults) significantly reduced otorrhea in chronic active otitis media 4. At 6 weeks, only 28% of children treated with trimethoprim-sulfamethoxazole had persistent otorrhea compared to 53% in the placebo group 4. The treatment effect was most pronounced with shorter courses and diminished after discontinuation 4.
For MRSA specifically, case series demonstrate successful treatment of acute otitis media with otorrhea using oral trimethoprim-sulfamethoxazole combined with topical antibiotics when patients failed conventional therapy 5. All six pediatric patients with CA-MRSA otorrhea achieved clinical resolution with this combination 5.
Rationale for Dual Coverage
MRSA coverage: Trimethoprim-sulfamethoxazole provides excellent activity against community-acquired MRSA, which is frequently resistant to fluoroquinolones and beta-lactams 1, 5
Proteus vulgaris coverage: Trimethoprim-sulfamethoxazole maintains activity against most Proteus species, though susceptibility testing is recommended 1
Topical augmentation: Topical antibiotics achieve high local concentrations and provide additional coverage for polymicrobial infections common in chronic otitis externa 5
Monitoring and Safety Considerations
Baseline assessment: Obtain complete blood count before initiating therapy, as thrombocytopenia can occur with prolonged treatment 2, 6
Monthly monitoring: Check hemogram monthly during prolonged therapy to detect hematological toxicity early 2
Renal function: Monitor serum creatinine, particularly in elderly patients who have increased risk of acute kidney injury with TMP-SMX 3
Common side effects: Gastrointestinal symptoms (nausea, vomiting, diarrhea) occur in approximately 9% of patients but are generally mild 4
Clinical Response Assessment
Early evaluation: Reassess at 6 weeks to determine if clinical improvement is occurring 4
Treatment duration: If significant improvement is noted at 6 weeks, consider completing 6-8 weeks total; if response is slower, extend to 12 weeks 4
Failure criteria: If no improvement by 6 weeks or worsening occurs, obtain repeat culture and susceptibility testing to guide alternative therapy 5
Important Caveats
Contraindications to avoid:
- Do not use in pregnancy at term or in nursing mothers due to kernicterus risk 3
- Avoid in patients with documented sulfa allergy 3
- Do not use in patients with G6PD deficiency due to hemolytic anemia risk 2
Resistance considerations:
- Pseudomonas aeruginosa, commonly isolated in chronic otitis externa, may not be adequately covered by trimethoprim-sulfamethoxazole alone 4
- If Pseudomonas is suspected or confirmed on culture, consider adding or switching to an anti-pseudomonal fluoroquinolone (ciprofloxacin) or aminoglycoside 4, 5
Dosing in higher-dose scenarios:
- While standard dosing (1 DS tablet twice daily) is effective for most SSTIs, some clinicians use 2 DS tablets twice daily for severe MRSA infections, though this higher dose has not shown superior efficacy in comparative studies 7
- For chronic otitis externa, the standard 1-2 DS tablets twice daily regimen is appropriate 1, 3