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

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

Answer: (C)

Niacin production is a key test used to distinguish tuberculous mycobacteria from non-tuberculous ones. Mycobacterium tuberculosis releases niacin into the medium, so the niacin test is positive, and its colonies are typically buff-colored—nonpigmented and light tan—on culture, a hue that can be observed after routine observation and light exposure. This combination of niacin positivity and buff-colored colonies fits M. tuberculosis.

Other options either lack niacin production (for example, some non-tuberculous species) or have pigment characteristics that don’t match the description. For instance, scotochromogenic organisms produce pigment in the dark, not as a buff coloration after light exposure, and other species have different colony traits or biochemical profiles.