Treatment of Mycobacterium fortuitum Infection
For M. fortuitum infections, treatment should be guided by in vitro susceptibility testing using at least two oral agents with documented activity for a minimum of 4 months for skin/soft tissue disease, 6 months for bone infections, and 12 months of culture-negative sputum for pulmonary disease. 1
Key Antimicrobial Susceptibility Profile
M. fortuitum isolates are typically susceptible to multiple oral agents, making them among the most treatable rapidly growing mycobacteria 1:
- Amikacin: 100% susceptible
- Fluoroquinolones (ciprofloxacin/ofloxacin): 100% susceptible
- Sulfonamides: 100% susceptible
- Imipenem: 100% susceptible
- Clarithromycin: 80% susceptible (but see critical caveat below)
- Doxycycline: 50% susceptible
- Cefoxitin: 50% susceptible
Critical Caveat About Macrolides
Despite appearing susceptible on testing, all M. fortuitum isolates contain an inducible erythromycin methylase erm(39) gene that confers macrolide resistance 1. Therefore, macrolides should be used with extreme caution and never as monotherapy, even when susceptibility testing suggests they would work. This is a common pitfall that can lead to treatment failure.
Treatment Algorithms by Site of Infection
Pulmonary Disease
- Regimen: At least two agents with documented in vitro activity
- Duration: Minimum 12 months of culture-negative sputum 1
- Agent selection: Based on patient tolerance and susceptibility results
- Monitoring: Frequent sputum cultures during therapy to assess response
Typical effective combinations include fluoroquinolone + sulfonamide, or fluoroquinolone + doxycycline (if susceptible).
Skin, Soft Tissue, and Bone Infections
For localized, less severe disease:
- Single oral agent based on susceptibilities (historically sulfonamide most commonly used)
- Cellulitis: ~10.6 weeks 2
- Osteomyelitis: 6-7 months 2
For serious or extensive disease:
- Initial phase: Parenteral therapy with amikacin (10-15 mg/kg daily) ± cefoxitin or imipenem for minimum 2-4 weeks until clinical improvement 1
- Continuation phase: Two oral agents based on susceptibilities
- Total duration: Minimum 4 months for soft tissue, 6 months for bone infections 1, 2
Surgical Considerations
Surgery is essential in these scenarios 1:
- Extensive disease with abscess formation
- Presence of foreign bodies (breast implants, catheters, prosthetic material)
- Areas where drug penetration is difficult
- Failure of medical therapy alone
In the historical series, 60% of patients required surgical debridement in addition to antibiotics 2.
Practical Treatment Examples
Recent case reports demonstrate successful regimens:
- Surgical site infections: Intravenous amikacin followed by extended oral therapy 3
- Soft tissue infection: Clarithromycin 500 mg twice daily + moxifloxacin 400 mg daily 4
- Hip joint infection: Azithromycin + levofloxacin + trimethoprim-sulfamethoxazole for 12 months 5
- Endocarditis: Amikacin-based combination with ciprofloxacin and clarithromycin 6
Monitoring and Follow-up
- Culture surveillance: Obtain cultures every 2-4 weeks initially to document response
- Expected timeline: Cultures typically become negative within 6 weeks for most infections (up to 14 weeks for sternal osteomyelitis) 2
- Serum drug levels: Consider monitoring aminoglycoside levels when used
- Follow-up duration: Minimum 12 months post-treatment to detect rare relapses 2
Important Clinical Pearls
Treatment success rates are excellent when guided by susceptibilities:
- 90% cure rate for M. fortuitum infections with appropriate therapy 2
- Drug resistance development is rare (only 1 patient in 76 in the largest series) 2
- Many mild cases (especially nail salon furunculosis) may resolve spontaneously or with single-agent therapy over several months 1
- Two-thirds of pulmonary disease patients may remain stable without antibiotics 7
The key to success is:
- Always obtain susceptibility testing 1
- Use combination therapy for serious infections
- Avoid macrolide monotherapy despite susceptibility results
- Ensure adequate surgical debridement when indicated
- Treat for sufficient duration based on infection site