Proteogenomic analysis identifies cell wall-associated signatures linked to oxacillin susceptibility in oxacillin-susceptible mecA-positive Staphylococcus aureus

Oxacillin-susceptible mecA -positive Staphylococcus aureus (OS-MRSA) presents a therapeutic challenge because it retains susceptibility to oxacillin despite harbouring the mecA gene. Here, we used integrated genomic, proteomic, expression, and phenotypic analyses to identify molecular features associated with this unusual phenotype. OS-MRSA isolates exhibited impaired biofilm formation and reduced fitness under oxacillin exposure. Whole-genome comparison of human and animal OS-MRSA isolates with MRSA identified 1,357 non-synonymous mutations across 498 genes, including 11 genes associated with cell wall-related functions, cell adhesion, and quorum sensing. Secretome analysis revealed reduced abundance of SceD and LtaS in OS-MRSA relative to MRSA, highlighting these cell wall-associated factors as candidate correlates of oxacillin susceptibility. In addition, a conserved upstream mutation in ltaS was identified across all OS-MRSA isolates, suggesting possible transcriptional modulation, and quantitative PCR confirmed differential expression of sceD and ltaS following oxacillin induction. Taken together, these findings support a model in which altered regulation of cell wall-associated processes may be linked to the OS-MRSA phenotype. However, because the study is based on comparative omics, expression profiling, and indirect phenotypic observations, these associations should be interpreted cautiously in the absence of direct lipoteichoic acid quantification and targeted mutant validation. Overall, this study provides a hypothesis-generating framework for understanding OS-MRSA adaptation to oxacillin exposure and identifies priorities for future functional investigation.