A Runyon group IV mycobacterium is identified in a post-surgical wound. It grows rapidly (within 7 days) on LJ medium, is non-photochromogen, and is resistant to most antibiotics except amikacin and clarithromycin. It is most likely:

• A Mycobacterium kansasii
• B Mycobacterium abscessus or M. fortuitum (rapid growers) ✓
• C Mycobacterium scrofulaceum
• D Mycobacterium avium complex (MAC)

Explanation

Runyon Group IV (rapid growers) consists of Mycobacterium fortuitum, M. chelonae, and M. abscessus — these grow in fewer than 7 days at 28–35°C. They are non-photochromogens and are associated with post-surgical/post-injection wound infections, catheter infections, and pulmonary disease in cystic fibrosis patients. Susceptibility testing is essential; amikacin and macrolides (clarithromycin/azithromycin) form the backbone of treatment. M. kansasii is Group II (scotochromogen); M. scrofulaceum is Group II; MAC is Group III (non-photochromogen but slow grower).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.