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Life sciences · Journal article
Clinical Microbiology and Infection · October 1, 2026
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BACKGROUND: Beyond mec-mediated resistance, Staphylococcus aureus has developed complex resistance mechanisms against anti-staphylococcal β-lactam-antibiotics. The scientific community lacks a unified consensus on the nomenclature/clinical implications of these mechanisms. OBJECTIVES: This position statement by members of Staphylococcus Working Groups of the International Society of Antimicrobial Chemotherapy and the European Society of Clinical Microbiology and Infectious Diseases proposes a set of names for various types of methicillin/oxacillin reduced susceptibility/resistance in S. aureus. CONTENT: The Working Groups propose the following categorisation: (1) Mec-mediated methicillin-resistant S. aureus (MRSA): This is due to mec (mecA or mecB or mecC) mediated resistance. They can exhibit heteroresistant expression to methicillin. (2) Methicillin-resistant S. aureus lacking mec (MRLM): Isolates are cefoxitin and/or oxacillin resistant but mec-negative, and do not show heteroresistance. They can be subdivided into 2a) hyperproduction of β-lactamase and 2b) modifications of natural penicillin-binding-proteins (PBP). Importantly, these mechanisms cannot easily be differentiated in routine laboratories. If the resistance mechanism is not fully characterised we suggest using the name methicillin-resistant lacking mec (MRLM). Isolates with hyperproduction of β-lactamase show low-level resistance to oxacillin, but most isolates test susceptible to cefoxitin. When the mechanism is characterised we propose they should be named hyper-beta-lactamase-producing-S. aureus (HBSA) instead of the hitherto used term borderline oxacillin-resistant S. aureus (BORSA). Isolates with resistance due to modifications of the PBP 1-4 in S. aureus, we propose to continue to name these modified S. aureus (MODSA). MODSA isolates in routine diagnostic laboratories most commonly present as cefoxitin-resistant. (3) Oxacillin susceptible-mec-positive S. aureus (OS-MRSA). Strains carry a mec gene, but harbor mutations, deletions or insertion rendering them phenotypically susceptible to oxacillin/cefoxitin. IMPLICATIONS: The proposed changes clarify methicillin resistance nomenclature in S. aureus, removing current ambiguity from clinical communication and literature. As novel methicillin resistance mechanisms emerge in S. aureus, the definition must be updated accordingly.