A 45-year-old immunocompetent patient with a history of hot-tub use develops multiple nodular skin lesions on the arms and trunk. A skin biopsy AFB culture at 30–33°C grows a scotochromogenic, rapidly-growing Mycobacterium. The most likely organism is:

• A Mycobacterium marinum
• B Mycobacterium fortuitum
• C Mycobacterium kansasii
• D Mycobacterium chelonae ✓

Explanation

M. chelonae is a rapidly growing mycobacterium (grows in <7 days) that is scotochromogenic (pigment in light and dark), prefers growth at 28–32°C, and is associated with hot-tub folliculitis/skin infections and post-procedure infections. M. marinum ('swimming pool granuloma') grows at 30–32°C but is photochromogenic and associated with fish tank exposure. M. fortuitum is also a rapid grower but is non-chromogenic. M. kansasii is a slow grower and photochromogen causing pulmonary disease.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.