A 60-year-old COPD patient (heavy smoker, no immunosuppression) develops progressive pulmonary disease with upper-lobe fibronodular infiltrates. BAL grows AFB that are acid-fast, rapidly growing (colonies within 7 days on LJ medium), non-chromogenic, and resistant to isoniazid and rifampicin but susceptible to amikacin and clarithromycin. Which organism is most likely?

• A Mycobacterium avium complex (MAC) — slowly growing NTM (3–6 weeks), not rapidly growing
• B Mycobacterium kansasii — photochromogenic, slowly growing, responds to rifampicin
• C Mycobacterium abscessus complex — rapidly growing NTM causing chronic pulmonary disease, particularly in COPD and bronchiectasis patients ✓
• D Mycobacterium fortuitum — rapidly growing, but primarily causes skin/soft tissue infections

Explanation

Rapidly growing mycobacteria (RGM) form colonies within 7 days; M. abscessus complex (M. abscessus, M. massiliense, M. bolletii) is the most clinically significant RGM causing chronic pulmonary disease in patients with COPD, cystic fibrosis, or bronchiectasis. It is intrinsically resistant to most antibiotics including INH, rifampicin, ethambutol, and all fluoroquinolones, but shows variable susceptibility to amikacin, imipenem, tigecycline, cefoxitin, and clarithromycin (inducible resistance via erm gene should be checked). MAC grows slowly (3–6 weeks); M. kansasii is photochromogenic and rifampicin-susceptible. M. fortuitum is a rapidly growing NTM but primarily causes skin/soft tissue/injection-site infections.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.