Methicillin-resistant Staphylococcus aureus (MRSA) isolates that carry mecA but are oxacillin susceptible (OS-MRSA; oxacillin minimum inhibitory concentration [MIC] ≤2 μg/mL) are increasingly reported worldwide in the past decade. In this study, an OS-MRSA isolate FC1102921 from an imported meat sample was characterized and the oxacillin resistance mutation frequency and mutant prevention concentration (MPC) to oxacillin was determined. Isolate FC1102921 was pvl negative and contained SCCmec V (5C2&5), the sequence type and spa type were identified as ST398-t034. The MPCs of isolate FC1102921 to oxacillin was 16 μg/mL. The resistance mutation frequency of isolate FC1102921 to oxacillin was 0.036%. The expression of mecA in isolate FC1102921 was detectable, but the mecA expression in the mutant isolate FC1102921m increased 13.8-folds. Our data indicated that S. aureus FC1102921 was not a hypermutator, and the high-resistance mutation frequency of S. aureus FC1102921 was specific to oxacillin and penicillin. SCCmec V region of isolate FC1102921 and FC1102921m was identical, functional blaI and blaR1 were identified in SCCmec region of both isolates, and no plasmid loss was identified in mutant isolate FC1102921m. To the best of our knowledge, this is the first report of the resistance mutation frequency and MPC study of an OS-MRSA ST398-t034 isolate. The high oxacillin MPC value and high oxacillin resistance mutation frequency was not related to a curable plasmid as previously reported, which further highlighted that an in-depth study should be carried out to clarify the mechanism and role of the OS-MRSA isolates in the emergence and transmission of MRSA ST398-t034.
Keywords: MRSA; mecA; oxacillin-susceptible.