Which organism typically demonstrates yellow to orange pigmentation of colonies when grown in the dark?

Which organism typically demonstrates yellow to orange pigmentation of colonies when grown in the dark?

• A. Mycobacterium scrofulaceum
• B. Mycobacterium fortuitum
• C. Mycobacterium kansasii
• D. Mycobacterium tuberculosis

Answer: (A)

Pigment production in the dark identifies scotochromogenic mycobacteria. These bacteria form yellow to orange colonies without requiring light exposure. Mycobacterium scrofulaceum is the classic example of a scotochromogen, so it reliably shows yellow-orange pigment when grown in the dark. In contrast, a photochromogen like Mycobacterium kansasii only develops pigment after light exposure, Mycobacterium fortuitum is a rapid grower and not the typical dark-pigment producer, and Mycobacterium tuberculosis usually remains nonpigmented. Thus, the organism most likely to exhibit yellow-orange pigment in the dark is Mycobacterium scrofulaceum.

Pigment production in the dark identifies scotochromogenic mycobacteria. These bacteria form yellow to orange colonies without requiring light exposure. Mycobacterium scrofulaceum is the classic example of a scotochromogen, so it reliably shows yellow-orange pigment when grown in the dark. In contrast, a photochromogen like Mycobacterium kansasii only develops pigment after light exposure, Mycobacterium fortuitum is a rapid grower and not the typical dark-pigment producer, and Mycobacterium tuberculosis usually remains nonpigmented. Thus, the organism most likely to exhibit yellow-orange pigment in the dark is Mycobacterium scrofulaceum.