A patient with HIV and CD4 of 40 cells/µL presents with disseminated infection. Blood cultures on BACTEC grow an organism after 10 days; colonies on LJ medium at 37°C appear as smooth buff-coloured colonies. The organism is pigmented only when grown in the dark. Niacin test is negative and nitrate reduction is negative. This most likely represents:
- A Mycobacterium tuberculosis
- B Mycobacterium kansasii
- C Mycobacterium marinum
- D Mycobacterium avium complex (MAC) ✓
Explanation
Mycobacterium avium complex (MAC) is the most common cause of disseminated NTM infection in advanced HIV (CD4 <50). MAC grows slowly on LJ at 37°C, producing non-chromogenic (scotochromogenic at room temperature but appears non-pigmented at 37°C), smooth, buff-coloured colonies. Niacin and nitrate reduction are both negative (unlike M. tuberculosis which is niacin positive and nitrate positive). M. kansasii is photochromogenic; M. marinum grows better at 30°C; M. tuberculosis has characteristic rough eugonic colonies. BACTEC detection in blood culture at approximately 10 days is typical for MAC in AIDS.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.