The distribution of plasmids that carry virulence and resistance genes in Staphylococcus aureus is lineage associated

A total of 21 plasmid rep gene families, of which 13 were novel to this study, were characterised using a previously proposed classification system. 243 sequenced plasmids were assigned to 39 plasmid groups that each possessed a unique combination of rep genes. We show some resistance genes (including ermC and cat) and virulence genes (including entA, entG, entJ, entP) were associated with specific plasmid groups suggesting there are genetic pressures preventing recombination of these genes into novel plasmid groups. Whole genome microarray analysis revealed that plasmid rep, resistance and virulence genes were associated with S. aureus lineages, suggesting restriction-modification (RM) barriers to HGT of plasmids between strains exist. Conjugation transfer (tra) complex genes were rare.This study argues that genetic pressures are restraining the spread of resistance and virulence genes amongst S. aureus plasmids, and amongst S. aureus populations, delaying the emergence of fully virulent and resistant strains.