Structural comparison of three types of staphylococcal cassette chromosomemec integrated in the chromosome in methicillin-resistant Staphylococcus aureus

The beta -lactam resistance gene mecA of Staphylococcus aureus is carried b y a novel mobile genetic element, designated staphylococcal cassette chromo some mec (SCCmec), identified in the chromosome of a Japanese methicillin-r esistant S, aureus (MRSA) strain. We now report identification of two addit ional types of mecA-carrying genetic elements found in the MRSA strains iso lated in other countries of the world. There were substantial differences i n the size and nucleotide sequences between the elements and the SCCmec, Ho wever, new elements shared the chromosomal integration site with the SCCmec , Structural analysis of the new elements revealed that they possessed all of the salient features of the SCCmec: conserved terminal inverted repeats and direct repeats at the integration junction points, conserved genetic or ganization around the mecA gene, and the presence of cassette chromosome re combinase (ccr) genes responsible for the movements of SCCmec, The elements , therefore, were considered to comprise the SCCmec family of staphylococca l mobile genetic elements together with the previously identified SCCmec, A mong 38 epidemic MRSA strains isolated in 20 countries, 34 were shown to po ssess one of the three typical SCCmec elements on the chromosome. Our findi ngs indicated that there are at least three distinct MRSA clones in the wor ld with different types of SCCmec in their chromosome.