A 32-year-old HIV-positive patient (CD4 count 45 cells/µL) has disseminated infection with fever and cervical lymphadenopathy. Blood culture on BACTEC system grows a slow organism. Ziehl-Neelsen stain of lymph node biopsy shows weakly acid-fast rods with a beaded pattern. Colonies on Lowenstein-Jensen medium are non-chromogenic at 37°C. The most likely organism is:

• A Mycobacterium kansasii
• B Mycobacterium marinum
• C Mycobacterium scrofulaceum
• D Mycobacterium avium-intracellulare complex (MAC) ✓

Explanation

MAC (Mycobacterium avium-intracellulare complex) is the most common cause of disseminated non-tuberculous mycobacterial (NTM) infection in advanced HIV disease (CD4 < 50 cells/µL). MAC is classified as a non-chromogen (Runyon group III), grows at 37°C, and produces non-pigmented colonies on LJ medium. M. kansasii is a photochromogen (produces pigment in light) and rarely causes disseminated disease. M. marinum grows at 30°C in water-related skin lesions. Disseminated MAC requires treatment with clarithromycin + ethambutol ± rifabutin.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.