 Mycobacterium szulgai is a photochromogen when grown at 24 degrees, and a scotochromogen at 37 degrees.In contrast, Runyon classifications III through IV are considered "nonphotochromogens", in that exposure to light does not make them produce pigment which they would not develop in dark growing conditions.Runyon II organisms (scotochromogens) are slow-growing and produce a yellow-orange pigment regardless of whether they are grown in the dark or the light.