A mycobacterial isolate from a sputum sample grows smooth colonies on Lowenstein-Jensen medium after three weeks. The colonies are cream-coloured when incubated in the dark but turn bright yellow-orange after brief exposure to light. This isolate belongs to which Runyon group?

• A Group III, non-photochromogens
• B Group II, scotochromogens
• C Group I, photochromogens ✓
• D Group IV, rapid growers

Explanation

Runyon's classification of atypical mycobacteria rests on growth rate and carotenoid pigment production. Photochromogens such as M. kansasii and M. marinum form pigment only after exposure to light. Scotochromogens like M. scrofulaceum produce yellow pigment even in the dark. Non-photochromogens including the MAC are non-pigmented. Rapid growers form mature colonies within seven days regardless of pigmentation. The light-dependent colour change described here is diagnostic of Group I.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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