Genetic organization of mecA and mecA-regulatory genes in epidemic methicillin-resistant Staphylococcus aureus from Australia and England.

The mecA gene that encodes methicillin resistance in Staphylococcus aureus may be regulated by the mecR1 and mecI genes, and this region has been referred to as the mec complex. An analysis of these regulatory genes in 35 epidemic methicillin-resistant S. aureus (MRSA) isolated in England and Australia has found that they contain three classes of mec complex. Firstly, the Class A mec complex with complete mecR1 and mecI genes. Secondly, a new variant of Class A, the Class A1 mec complex, with a 166 bp deletion in the membrane-spanning domain of mecR1 and a complete mecI gene. Thirdly, the Class B mec complex, in which the penicillin-binding domain of mecR1 and the whole mecI gene are deleted by the insertion of a partial sequence of IS1272. Seven MRSA isolated in England and Australia over different time periods had the Class A mec complex. However, the isolates did not have closely related pulsed-field gel electrophoresis (PFGE) patterns. The Class A1 mec complex was found in 12 Australian isolates and the English epidemic MRSA, EMRSA-1. All these organisms were isolated in the early 1980s and had closely related PFGE patterns. The Class B mec complex region was found in nine EMRSA and seven Australian MRSA isolated over the period from the 1970s to 2000. These isolates had related PFGE patterns. The mecA region was also compared in the isolates and all but two of the isolates had an XbaI restriction site. These results support the global spread of epidemic clones and confirm the close relationship between the Australian and English MRSA strains.