A 35-year-old HIV-positive man (CD4 count 80 cells/µL) presents with fever, weight loss, and a positive blood culture growing acid-fast bacilli. The organism grows on Löwenstein-Jensen medium as smooth, cream-coloured colonies at 37°C after 3 weeks and is negative for niacin accumulation and nitrate reduction. It is resistant to isoniazid and rifampicin at standard concentrations. Which organism is most likely?

• A Mycobacterium tuberculosis
• B Mycobacterium kansasii
• C Mycobacterium avium-intracellulare complex (MAC) ✓
• D Mycobacterium fortuitum

Explanation

Mycobacterium avium-intracellulare complex (MAC) is the most common disseminated non-tuberculous mycobacterial infection in advanced HIV disease (CD4 <50 cells/µL). MAC is niacin-negative and nitrate reduction-negative (distinguishing it from M. tuberculosis, which accumulates niacin and reduces nitrates). MAC is inherently resistant to first-line anti-TB drugs and requires a combination of clarithromycin, ethambutol, and rifabutin. M. kansasii is photochromogenic. M. fortuitum is a rapid grower (within 7 days).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.