An isolate of an atypical mycobacterium grown on Löwenstein-Jensen medium develops a deep yellow pigment only after exposure to light. In the Runyon classification of nontuberculous mycobacteria, such an organism belongs to which group?

• A Group III nonphotochromogens
• B Group II scotochromogens
• C Group I photochromogens ✓
• D Group IV rapid growers

Explanation

Runyon classified slow-growing nontuberculous mycobacteria by carotenoid pigmentation. Group I photochromogens produce yellow-orange pigment only after photoinduction; Mycobacterium kansasii is the prototype. Group II scotochromogens make pigment even in the dark (M. scrofulaceum), and group III organisms are unpigmented (M. avium-intracellulare). Pigment appearing only after light exposure is therefore diagnostic of the photochromogen group.

Reference: Jawetz, Melnick and Adelberg's Medical Microbiology, 28th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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