In vitro activity and resistance rates of topical antimicrobials fusidic acid, mupirocin and ozenoxacin against skin and soft tissue infection pathogens obtained across Canada (CANWARD 2007–18)

Background Current antimicrobial susceptibility/resistance data versus skin and soft tissue infection (SSTI) pathogens help to guide empirical treatment using topical antimicrobials. Objectives To assess the in vitro activity and resistance rates of fusidic acid, mupirocin, ozenoxacin and comparator agents against pathogens isolated from patients with SSTIs in Canada. Methods SSTI isolates of MSSA (n = 422), MRSA (n = 283) and Streptococcus pyogenes (n = 46) obtained from CANWARD 2007–18 were tested using CLSI broth microdilution. Fusidic acid low-level resistance was defined as an MIC of ≥2 mg/L and high-level resistance as an MIC ≥512 mg/L. Mupirocin high-level resistance was defined as an MIC ≥512 mg/L and low-level resistance was an MIC of 2–256 mg/L. Results Low-level and high-level fusidic acid resistance in MSSA was 10.9% and 1.7%, respectively. Low-level and high-level fusidic acid resistance in MRSA was 10.6% and 3.5%, respectively. High-level mupirocin resistance was identified in 1.4% of MSSA and 14.1% of MRSA, respectively. Versus MSSA, ozenoxacin demonstrated MIC50 and MIC90 of 0.004 and 0.25 mg/L, respectively. Against MRSA, ozenoxacin inhibited all isolates at an MIC of ≤0.5 mg/L, including isolates with ciprofloxacin MICs >2 mg/L, clarithromycin-resistant, clindamycin-resistant, high-level fusidic acid-resistant and high-level mupirocin-resistant isolates. Conclusions We conclude that fusidic acid low-level resistance exceeded 10% for both MSSA and MRSA while fusidic acid high-level resistance was ≤3.5%. Mupirocin high-level resistance exceeded 10% in MRSA. Ozenoxacin is active versus SSTI pathogens including MRSA resistant to fluoroquinolones, macrolides, clindamycin, fusidic acid and mupirocin.