Trimethoprim–sulfamethoxazole susceptibility in beta-hemolytic streptococci in a Canadian tertiary center

OBJECTIVES: Beta-hemolytic streptococci (BHS) are responsible for a large proportion of skin and soft tissue infections. Since the 1940s, studies evaluating BHS resistance to trimethoprim-sulfamethoxazole (TMP-SMX) have produced inconsistent results, limiting the use of this agent for these infections. To our knowledge, the true resistance profile of BHS to TMP-SMX has not been assessed in Canada for many years. Our primary objective was therefore to describe the contemporary susceptibility of BHS isolates to TMP-SMX in our mixed urban-rural region served by a tertiary care center in Quebec, Canada. Susceptibility to tetracycline, levofloxacin, erythromycin, and clindamycin was also assessed. METHODS: BHS isolates from various clinical specimens were identified at our tertiary care hospital laboratory, which serves as the reference laboratory for the region, between December 30th, 2024, and January 30th, 2025. Antibiotic susceptibility was assessed using disk diffusion according to Clinical and Laboratory Standards Institute (CLSI) standards for tetracycline, levofloxacin, erythromycin, and clindamycin (including testing for inducible resistance - D-test), and according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) standards for TMP-SMX. RESULTS: Among 151 BHS isolates identified in January 2025, 99% of isolates were susceptible to TMP-SMX, 98% to levofloxacin, 74% to tetracycline, and 72% to clindamycin. CONCLUSION: In vitro susceptibility of BHS isolates to TMP-SMX at our institution in January 2025 was 99%. The clinical (in vivo) effectiveness of TMP-SMX for proven or suspected BHS infections remains to be determined.