Genetic relatedness between methicillin-susceptible and methicillin-resistant Staphylococcus aureus: results of a national survey.

OBJECTIVES Surveillance of hospital-acquired methicillin-resistant Staphylococcus aureus (HA-MRSA) infections has shown the emergence and spread of several epidemic MRSA clones over the past 10 years in Belgium. Whether these clones have been imported from abroad or else have arisen locally via staphylococcal cassette chromosome mec (SCCmec) acquisition by successful methicillin-susceptible S. aureus (MSSA) clones is unknown. METHODS We determined by PFGE, spa typing, multi-locus sequence typing (MLST) and agr group analysis the genetic relatedness of 103 MSSA and 511 MRSA strains from a national survey of patients admitted to 112 Belgian hospitals in 2003. RESULTS The 103 MSSA strains presented very diverse genetic backgrounds, they were distributed into 40 distinct PFGE types and clustered in 15 distinct MLST CCs. Up to 45% harboured the same genotype as five major epidemic HA-MRSA clones. These MRSA clones all harbour a type IV SCCmec element. CONCLUSIONS These findings are consistent with multiple recent acquisitions of the more mobile type IV SCCmec by MSSA and suggest that certain genetic backgrounds are conferring a selective advantage, regardless of the resistance profile. However, since the predominant MSSA and MRSA lineages identified in Belgium are disseminated worldwide, importation of epidemic MRSA strains remains an alternative hypothesis.