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Open Access Highly Accessed Research article

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

Alex J McCarthy* and Jodi A Lindsay

Author Affiliations

Centre for Infection, Division of Clinical Sciences, St George’s, University of London, London, UK

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BMC Microbiology 2012, 12:104  doi:10.1186/1471-2180-12-104

Published: 12 June 2012

Abstract

Background

Staphylococcus aureus is major human and animal pathogen. Plasmids often carry resistance genes and virulence genes that can disseminate through S. aureus populations by horizontal gene transfer (HGT) mechanisms. Sequences of S. aureus plasmids in the public domain and data from multi-strain microarrays were analysed to investigate (i) the distribution of resistance genes and virulence genes on S. aureus plasmids, and (ii) the distribution of plasmids between S. aureus lineages.

Results

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.

Conclusion

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.