Structural Comparison of Three Types of Staphylococcal Cassette Chromosome mec Integrated in the Chromosome in Methicillin-Resistant Staphylococcus aureus

Abstract

The β-lactam resistance gene mecA of Staphylococcus aureus is carried by a novel mobile genetic element, designated staphylococcal cassette chromosome mec (SCC mec ), identified in the chromosome of a Japanese methicillin-resistant S. aureus (MRSA) strain. We now report identification of two additional types of mecA -carrying genetic elements found in the MRSA strains isolated in other countries of the world. There were substantial differences in the size and nucleotide sequences between the elements and the SCC mec . However, new elements shared the chromosomal integration site with the SCC mec . Structural analysis of the new elements revealed that they possessed all of the salient features of the SCC mec : conserved terminal inverted repeats and direct repeats at the integration junction points, conserved genetic organization around the mecA gene, and the presence of cassette chromosome recombinase ( ccr ) genes responsible for the movements of SCC mec . The elements, therefore, were considered to comprise the SCC mec family of staphylococcal mobile genetic elements together with the previously identified SCC mec . Among 38 epidemic MRSA strains isolated in 20 countries, 34 were shown to possess one of the three typical SCC mec elements on the chromosome. Our findings indicated that there are at least three distinct MRSA clones in the world with different types of SCC mec in their chromosome.