A 55-year-old man with chronic cough and an apical cavitary lesion has sputum cultured on Lowenstein-Jensen medium. Colonies appear buff initially but turn lemon-yellow when the culture plate is exposed to light for a few hours. Which organism is most likely?

• A Mycobacterium kansasii ✓
• B Mycobacterium fortuitum
• C Mycobacterium avium complex
• D Mycobacterium scrofulaceum

Explanation

Photochromogens (Runyon group I) form colonies that develop yellow-orange carotenoid pigment only after exposure to light; M. kansasii is the classic example and causes pulmonary disease closely mimicking tuberculosis, especially in older men with pre-existing lung disease. M. scrofulaceum is a scotochromogen, pigmented even in the dark. MAC is a non-chromogen and M. fortuitum is a rapid grower.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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