Morphology: **Rapidly growing mycobacterium (RGM)** — grows in 3–7 days unlike slow-growing TB/MAC. Acid-fast bacilli. Environmental (water, soil). Three subspecies: abscessus, massiliense, bolletii (massiliense lacks functional erm gene → more macrolide-susceptible).
Typical drugs
- #1Azithromycin— **500 mg PO daily** — backbone but watch for inducible erm-mediated resistance (abscessus subspecies). Subspecies identification + erm sequencing essential.
- #2Clofazimine— 100 mg PO daily — combination component for cure regimens.
- #3Meropenem— 2 g IV q8h × 2–4 weeks induction (intensive phase). Imipenem alternative.
- #4Linezolid— 600 mg PO BID (often dose-reduced to daily after months due to cytopenias). Amikacin (not seeded) is also a key induction-phase component.
Empiric therapy when resistant
Notoriously difficult to cure. Multi-drug regimens (3–5 drugs) × 12+ months are standard. Even with treatment, cure rates 25–50%. Combine surgical resection + medical therapy for localized cavitary disease.
Resistance mechanisms
altered-target
Inducible erm(41) gene → macrolide resistance
Example: Subspecies abscessus (functional erm) often macrolide-resistant after 14-day exposure even if initial susceptibility looks good. Subspecies massiliense (non-functional erm) typically remains susceptible. **Subspecies + erm sequencing drives regimen.**
Resistance notes
Intrinsically resistant to most TB drugs (RIF, INH, EMB at standard doses). Susceptibility testing + subspecies identification essential before regimen design.
Pearls
Most antibiotic-resistant of the NTMs. Pulmonary disease in CF + bronchiectasis + COPD. Skin + soft tissue infections after cosmetic procedures, tattoos, surgery, fish/water exposure — derm-relevant. Treatment is brutal — intensive IV phase (2–4 weeks of meropenem ± amikacin ± tigecycline) → maintenance with macrolide (if susceptible) + clofazimine + linezolid for 12+ months. Subspecies massiliense has better outcomes than abscessus (functional vs non-functional erm). Surgical resection for localized disease often combined with medical therapy. Lung transplant contraindication in some centers (recurrence high).
References
- ATS/ERS/ESCMID/IDSA Nontuberculous Mycobacterial Guidelines (2020)
- Floto — CF Foundation NTM Guidelines (2016)