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

<i>Mycobacterium smegmatis</i> 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 <i>M. smegmatis</i> from 1998 to 2023 at our institution to evaluate the clinical significance of recovering this mycobacterium. Antimicrobial susceptibility patterns were also determined. Twenty-two <i>M. smegmatis</i> isolates were identified from 17 patients, 12 of whom met criteria for clinical chart review. <i>M. smegmatis</i> 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 <i>M. smegmatis</i> should be correlated with clinical presentation as it may represent a true infection.