Ten days after cardiac surgery, a 60-year-old man develops discharging sternal wound infection. Pus culture on blood agar shows colonies within 4 days that are acid-fast on Ziehl-Neelsen stain. Which organism is the most likely cause of this rapidly growing postoperative infection?
- A Mycobacterium avium complex
- B Mycobacterium fortuitum ✓
- C Mycobacterium kansasii
- D Mycobacterium xenopi
Correct answer: B. Mycobacterium fortuitum
Explanation
Runyon Group IV rapid growers form visible colonies within 7 days and classically cause surgical site and post-injection abscesses, catheter-related infections, and traumatic wound infections. M. fortuitum and M. chelonae-abscessus are the prototypes. M. avium complex and M. xenopi are slow-growing nonphotochromogens taking weeks to appear, and M. kansasii is a slow-growing photochromogen. Rapid growers are also relatively resistant to standard antitubercular drugs and often need macrolide-based combinations.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.