Morphology: Acid-fast bacilli (Ziehl-Neelsen). Group includes **M. avium + M. intracellulare** (and M. chimaera, environmental contaminant from heater-cooler units). Environmental — soil, water, biofilms. Slow-growing (3–4 weeks to grow).
Typical drugs
- #1Azithromycin— **500 mg PO daily** (or 600 mg 3× weekly) — backbone of MAC pulmonary regimen. Clarithromycin alternative.
- #2Ethambutol— 15 mg/kg PO daily — prevents macrolide resistance emergence.
- #3Rifampin— 10 mg/kg PO daily — third drug in regimen.
- #4Rifabutin— 300 mg PO daily — substitute for rifampin in HIV co-infection (less ART interaction).
Empiric therapy when resistant
Macrolide-resistant MAC pulmonary: linezolid + clofazimine + ethambutol regimens, possible bedaquiline; specialist-managed. Often combined with surgical resection for localized disease.
Resistance mechanisms
altered-target
23S rRNA mutations → macrolide resistance
Example: Single biggest risk for treatment failure. **NEVER use macrolide monotherapy** for MAC — always with ethambutol + rifampin to prevent resistance.
Resistance notes
Macrolide monotherapy fails — always combo. Susceptibility testing standard for treatment failure cases.
Pearls
MAC pulmonary disease comes in two phenotypes: fibrocavitary (older smokers, COPD — resembles TB) + nodular bronchiectatic ("Lady Windermere syndrome" — older non-smoking women, R middle lobe + lingula bronchiectasis). MAC disseminated in HIV/AIDS (CD4 <50) — fevers, weight loss, hepatosplenomegaly, cytopenias. Now rare with ART; azithromycin 1200 mg weekly prophylaxis historically for CD4 <50 but increasingly deferred for early ART initiation. MAC pulmonary regimen: macrolide + ethambutol + rifampin × 12 months after culture conversion (typically 18+ months total). Macrolide monotherapy NEVER — drives resistance + treatment failure. Surgical resection for localized cavitary disease or failed medical therapy.