Morphology: Acid-fast bacillus. Slow-growing photochromogen (yellow pigment with light). Environmental — tap water, soil. Most virulent of the slow-growing NTMs; **closest to MTB clinically**.
Typical drugs
- #1Rifampin— 10 mg/kg PO daily — backbone of standard regimen.
- #2Ethambutol— 15 mg/kg PO daily — second drug in standard regimen.
- #3Isoniazid— 5 mg/kg PO daily (max 300 mg) + pyridoxine — third drug in standard regimen (replacing classic INH-EMB-RIF triple).
- #4Azithromycin— Alternative third drug; preferred over INH per ATS 2020 in selected cases (3-drug regimen RIF + EMB + macrolide).
Empiric therapy when resistant
Rifampin-resistant: substitute macrolide for rifampin + add quinolone. Specialist consult.
Resistance mechanisms
altered-target
rpoB mutations → rifampin resistance
Example: Rifampin-resistant kansasii needs alternative regimens — macrolide + isoniazid + ethambutol + maybe FQ; specialist-managed.
Resistance notes
Most kansasii isolates susceptible to RIF + EMB + macrolide. Resistance rare in untreated patients.
Pearls
Most TB-like NTM — cavitary upper-lobe pulmonary disease, weight loss, cough. Standard treatment: rifampin + ethambutol + (INH or macrolide) × 12 months after culture conversion (typically ~12–18 months total). Better outcomes than MAC or M. abscessus — most patients cured. Photochromogen pigment + clinical resemblance to TB — distinguish with proper culture + DST.
References
- ATS/ERS/ESCMID/IDSA Nontuberculous Mycobacterial Guidelines (2020)