Abstract Objectives The present study examines methicillin-resistant Staphylococcus aureus isolates lacking mec (MRLM) phenotypic and genotypic traits and clinical features of patients. Methods Between October 2021 and February 2023, 13 MRLM isolates were identified among 867 S. aureus clinical isolates and subjected to WGS. Results Ten of the 13 MRLMs were resistant to cefoxitin (MICs 4 mg/L), including two that were also resistant to oxacillin. The remaining three isolates were susceptible to both cefoxitin and oxacillin and exhibited typical growth on CHROMagar MRSA. Seven of the 13 isolates harboured the blaZ gene, but the hyperproduction of the β-lactamase encoded by this gene was not predicted. All isolates exhibited low MICs for cefazolin, ceftriaxone, imipenem, and meropenem and demonstrated susceptibility to the fifth-generation cephalosporin ceftobiprole. Ten STs/CCs, encompassing ST5, ST8, ST20, ST25, ST30, ST45 (3 isolates), ST291, ST398 (2 isolates), CC15 and CC22, identified among isolates, were within the range of previously reported MRLM clones. A multitude of previously reported or undocumented mutations/substitutions in PBPs, the pbp4 promoter, GdpP, YjbH and AcrB were identified. Four isolates had truncations or insertions in GdpP. The exploration of clinical features suggests the potential coexistence of MRLM and MSSA populations within each patient in the absence of antimicrobial selective pressure. Conclusions The importance of accurate differentiation of MRLMs from MRSAs and MSSAs in routine laboratory testing is underscored by our results, which may allow in-depth investigation of the true epidemiology and clinical implications of MRLMs and the actual efficacy of β-lactams against them.
Natori et al. (Wed,) studied this question.