Which Mycobacterium appears as buff-colored colonies after exposure to light and is niacin positive?

• A. M. bovis
• B. M. ulcerans
• C. M. tuberculosis
• D. M. scrofulaceum

Answer: (C)

Biochemical testing together with colony appearance on culture is used to tell Mycobacterium species apart. Mycobacterium tuberculosis forms pale buff (cream-colored) colonies on culture media like Löwenstein-Jensen and, importantly, is positive for niacin production. That niacin positive result is a classic way to distinguish it from closely related species, especially M. bovis, which is niacin negative.

So the combination of buff-colored colonies and niacin positivity is characteristic of tuberculosis-causing M. tuberculosis. Other listed organisms either do not produce niacin (making them niacin negative) or show different pigment patterns, so they don’t fit this specific profile.