What is the recommended treatment for Mycobacterium fortuitum infections?

Treatment of Mycobacterium fortuitum Infections

For Mycobacterium fortuitum infections, treatment should include at least two antibiotics with in vitro activity against the clinical isolate for a duration of 4 months for skin/soft tissue infections and 6 months for bone infections. 1

Antibiotic Susceptibility Profile

M. fortuitum isolates typically show susceptibility to multiple antibiotics with the following susceptibility rates:

• Amikacin (100%)
• Ciprofloxacin/ofloxacin (100%)
• Sulfonamides (100%)
• Imipenem (100%)
• Clarithromycin (80%)
• Cefoxitin (50%)
• Doxycycline (50%)

Treatment Algorithm by Infection Site

Pulmonary Disease

1. Obtain drug susceptibility testing before initiating therapy
2. Administer at least two agents with in vitro activity for at least 12 months after achieving negative sputum cultures 1
3. Monitor monthly sputum cultures during initial treatment
4. Note: Many patients (approximately two-thirds) with M. fortuitum pulmonary disease remain stable without antibiotic treatment 2

Skin, Soft Tissue, and Wound Infections

1. Surgical debridement of infected tissue (essential component)
2. Combination antibiotic therapy for a minimum of 4 months 1
3. Recommended combinations (based on susceptibility):
   • Ciprofloxacin (750 mg orally twice daily) plus an aminoglycoside (amikacin) 1
   • Consider adding clarithromycin (500 mg twice daily) 1

Bone Infections (Osteomyelitis)

1. Aggressive surgical debridement
2. Combination antibiotic therapy for a minimum of 6 months 1
3. Consider longer treatment duration if clinical response is suboptimal at 6 months

Catheter-Related or Foreign Body Infections

1. Remove infected catheter or foreign body (essential for recovery) 1
2. Combination antibiotic therapy for a minimum of 4 months

Important Considerations

Macrolide Use Caution

Despite apparent in vitro susceptibility to clarithromycin in approximately 80% of isolates, M. fortuitum contains an inducible erythromycin methylase erm(39) gene that can confer resistance to macrolides. Therefore, macrolides should be used with caution and not as monotherapy. 1

Surgical Intervention

Surgery is generally indicated in cases with:

• Extensive disease
• Abscess formation
• Difficult-to-treat infections
• Foreign body involvement 1, 3

Monitoring Response

• For pulmonary disease: Monthly sputum cultures until conversion to negative
• For skin/soft tissue infections: Clinical improvement with resolution of drainage and inflammation

Special Situations

Nail Salon Furunculosis

Many cases of M. fortuitum furunculosis associated with nail salons either heal spontaneously or respond to oral antibiotic therapy, though disease may last several months to a year. 1

Disseminated Disease

More aggressive and prolonged therapy may be necessary, with close monitoring for relapse. Consider adding a third agent based on susceptibility testing.

Treatment Pitfalls to Avoid

1. Inadequate duration of therapy: Premature discontinuation before achieving complete resolution increases risk of relapse
2. Monotherapy: Using single-agent therapy increases risk of developing resistance
3. Failure to remove foreign bodies: Retention of infected implants or catheters almost always leads to treatment failure
4. Overlooking surgical intervention: Antibiotics alone are often insufficient without adequate debridement
5. Ignoring susceptibility testing: Treatment should always be guided by in vitro susceptibility results 1, 4

By following this approach with appropriate antibiotic selection based on susceptibility testing and adequate surgical intervention when indicated, successful outcomes can be achieved in the majority of M. fortuitum infections.