Treatment of Staphylococcus aureus infections is increasingly challenged by antimicrobial resistance. High‑level β‑lactam resistance in methicillin‑resistant S. aureus (MRSA) requires two mechanisms: the acquisition of mecA encoding penicillin‑binding protein PBP2A, and mutations in chromosomal genes that potentiate resistance. In MRSA, clinically common potentiator mutations map to the rpoB and rpoC genes encoding for the β and β′ subunits of RNA polymerase. However, it is still unclear how these contribute to high-level resistance. Interestingly, RNA polymerase mutations are frequently selected across bacteria in response to diverse stresses beyond antibiotic exposure. My work has revealed that ~40% of clinical methicillin‑sensitive S. aureus (MSSA) strains harbour modified rpoB or rpoC alleles. This unexpectedly high prevalence suggests that these mutations may confer adaptive benefits independent of methicillin resistance. I have demonstrated the impact of rpoB and rpoC mutations on clinically relevant traits including biofilm formation, bacteriophage resistance, and internalisation into lung epithelial cells. Thus, there are at least two sub-populations of S. aureus with markedly different phenotypic properties seemingly co-existing, allowing a swift adaptation to a changing environment. The prevalence of RNA polymerase mutations also suggests that these may predate the acquisition of the mec element. Hence, I have initiated a high‑throughput approach to determine which RNA polymerase variants circulating in MSSA have the capacity to potentiate high‑level β‑lactam resistance upon mec acquisition. This work highlights a previously underappreciated role for RNA polymerase plasticity in S. aureus adaptation, pathogenicity, and the evolutionary origins of antimicrobial resistance.