DNA microarray analysis reveals variability of accessory genes among isolates recovered during an MRSA outbreak

Ten MRSA isolates recovered during a large outbreak occurring at the Lausanne University Hospital (Switzerland) were characterized using a DNA microarray (StaphyType, Alere Technologies, Germany) that targets approximately 180 genes including several resistance and virulence genes. Each isolate belonged to the South German clone (ST-228-SCC mec I) and had been recovered between September 2008 and December 2009. As expected during the clonal dissemination of a strain, the 10 isolates shared identical presence/absence for most of the c.a. 180 genes tested. Nevertheless, variation was observed for several resistance and virulence genes. These included the beta lactamase operon genes ( blaZ , blaI , blaR ), genes involved in the resistance to Trimethoprim and Mupirocin ( dfrA and mupR , respectively), a gene encoding unspecific efflux pump conferring resistance to a variety of antiseptic such as chlorhexidine ( qacA ), and genes potentially involved in virulence ( lukX , lukY , aur ). This variability affected at least 5 of the ten isolates. Our results indicate the gain/loss of resistance and virulence genes during a local outbreak, suggesting that the biological characteristics of the strain might vary through time.