What is the most effective antibiotic regimen for a skin infection caused by rapidly growing nontuberculous Mycobacterium such as Mycobacterium fortuitum?

Treatment of Rapidly Growing Mycobacterium Skin Infections

For skin infections caused by rapidly growing mycobacteria like M. fortuitum, surgical debridement followed by combination antibiotic therapy with ciprofloxacin (750 mg orally twice daily) plus an aminoglycoside (amikacin) or imipenem for at least 4 months is the recommended approach. 1

Initial Management Algorithm

Surgical intervention is the critical first step:

• Perform surgical debridement of infected tissue 1
• Remove any foreign bodies (implants, catheters) as this is essential to recovery 1
• Obtain tissue for mycobacterial culture and drug susceptibility testing before starting antibiotics 2, 3

Empiric antibiotic therapy while awaiting culture results:

• Start amikacin plus either cefoxitin or imipenem for at least 2 weeks or until clinical improvement 2
• Continue for minimum 2 weeks pending susceptibility results 2

Definitive Antibiotic Selection Based on Species

For M. fortuitum specifically:

M. fortuitum has excellent susceptibility to multiple oral agents (100% susceptibility to amikacin, ciprofloxacin/ofloxacin, sulfonamides, and imipenem) 2, 3, making it easier to treat than other rapidly growing mycobacteria.

Recommended regimens:

• Ciprofloxacin 750 mg orally twice daily PLUS an aminoglycoside (amikacin) OR imipenem 1
• Alternative: Any two-drug combination based on in vitro susceptibility 1
• Some clinicians add clarithromycin 500 mg orally twice daily 1

For M. chelonae:

• Clarithromycin 500 mg twice daily is highly effective (100% susceptibility) 1
• Add tobramycin (preferred over amikacin for M. chelonae) or imipenem as second agent 1
• Note: M. chelonae is uniformly resistant to cefoxitin, unlike M. fortuitum 1

For M. abscessus:

• This species is the most difficult to treat among rapidly growing mycobacteria 1
• Requires macrolide (clarithromycin) plus parenteral agents (amikacin, cefoxitin, or imipenem) 1
• Surgical resection combined with multidrug therapy is often necessary 1

Critical Macrolide Warning

Exercise extreme caution with clarithromycin for M. fortuitum despite apparent susceptibility:

• All M. fortuitum isolates contain an inducible erythromycin methylase erm(39) gene that confers macrolide resistance 2, 3
• Despite 80% showing "susceptible" MICs in vitro, macrolides should never be used as monotherapy 2, 3
• If using clarithromycin, always combine with at least one other active agent 2, 3

Treatment Duration

Minimum treatment durations based on infection site:

• Skin and soft tissue infections: 4 months minimum with at least two active agents 1, 2, 3
• Bone infections: 6 months minimum 1, 3
• If response to initial 6 months is suboptimal, extend therapy up to 2 years 1

Monitoring and Follow-up

Track clinical response carefully:

• Cultures typically become negative within 6 weeks of appropriate therapy 3
• If cultures remain positive after 3 months, repeat susceptibility testing to detect acquired resistance 3
• Follow cultures to document response to therapy 3

Common Pitfalls to Avoid

Do not treat empirically for tuberculosis - standard antituberculous drugs are ineffective against rapidly growing mycobacteria 4, 5

Do not rely on antibiotics alone for extensive disease - 60-74% of patients require surgical intervention for cure 3

Do not use single-agent therapy - always use combination therapy to prevent resistance development 1, 2

Do not assume all rapidly growing mycobacteria have the same susceptibility patterns - M. chelonae, M. fortuitum, and M. abscessus have distinct resistance profiles requiring species-specific therapy 1