Rapidly growing knowledge of Mycobacterium smegmatis: A case series and review of antimicrobial susceptibility patterns

Mycobacterium smegmatis is a rapidly growing nontuberculous mycobacterium that is rarely isolated from clinical specimens and is frequently considered to be a contaminant. We conducted a retrospective review of mycobacterial cultures positive for M. smegmatis from 1998 to 2023 at our institution to evaluate the clinical significance of recovering this mycobacterium. Antimicrobial susceptibility patterns were also determined. Twenty-two M. smegmatis isolates were identified from 17 patients, 12 of whom met criteria for clinical chart review. M. smegmatis was deemed a cause of infection in 5/5 isolates from skin or soft tissue, 3/3 from bone, 1/1 from blood, and 0/3 from respiratory specimens. All cases thought to be significant were treated with at least 2 active agents for periods varying from 2 weeks up to 8 months. 18 isolates had antimicrobial susceptibility testing performed and all were susceptible to doxycycline, imipenem, linezolid, moxifloxacin, trimethoprim/sulfamethoxazole, and tobramycin while all isolates were resistant to clarithromycin. When recovered in culture, the presence of M. smegmatis should be correlated with clinical presentation as it may represent a true infection.