A sputum isolate from a 55-year-old farmer with upper lobe cavitary disease produces buff colonies on Lowenstein-Jensen medium after 3 weeks. When the same slant is exposed to light for a few hours, the colonies develop a bright yellow-orange pigment. To which Runyon group does this organism belong?

• A Group I, photochromogen ✓
• B Group II, scotochromogen
• C Group III, nonphotochromogen
• D Group IV, rapid grower

Explanation

Pigment appearing only after light exposure defines a photochromogen, Runyon Group I, and in pulmonary disease the classic example is Mycobacterium kansasii. Scotochromogens such as M. scrofulaceum produce pigment even in the dark, nonphotochromogens such as M. avium complex produce no clinically relevant carotenoid pigment, and rapid growers form colonies within 7 days. M. kansasii is a recognized cause of TB-like lung disease in older adults and responds well to rifampicin-containing therapy.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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