According to the Runyon classification of nontuberculous mycobacteria, organisms that grow mature colonies within 7 days on solid media are placed in which group?

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

Explanation

Runyon grouped NTM by growth rate and pigment production. Rapid growers form visible colonies on Löwenstein-Jensen medium in under 7 days and include Mycobacterium fortuitum, M. chelonae, and M. abscessus. Groups I to III are slow growers taking more than 7 days. Pigment production distinguishes the slow-growing groups: photochromogens pigment only after light exposure, scotochromogens pigment in the dark, and nonchromogens never pigment.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.