Novel type of staphylococcal cassette chromosome mec in a methicillin-resistant Staphylococcus aureus strain isolated in Sweden

Abstract

We identified a novel type of staphylococcal cassette chromosome mec (SCCmec) element carried by methicillin-resistant Staphylococcus aureus (MRSA) strain JCSC6082 isolated in Sweden. The SCCmec element was demarcated by characteristic nucleotide sequences at both ends and was integrated at the 3' end of orfX. The element carried a novel combination of a type 5 ccr gene complex and class C1 mec gene complex. The J regions of the element were homologous to those of the SCCmercury element of S. aureus strain 85/2082, with nucleotide identity greater than 99%. However, the novel SCCmec element from JCSC6082 did not carry the mer operon nor Tn554, suggesting that evolution to SCCmec could have been from a common ancestor by acquisition of the class C1 mec gene complex. The novel SCCmec element from JCSC6082 was flanked by a novel SCC-like chromosome cassette (CC6082), which was demarcated by two direct repeats and could be excised from the chromosome independently of the SCCmec element. Our data suggest that novel SCCmec elements can be generated on the staphylococcal chromosome through the recombination between extant SCC elements and mec gene complexes.

FIG. 1.

Structural comparisons of the novel SCCmec element and a chromosome cassette identified from JCSC6082 isolated in Sweden, SCCmercury, and type III SCCmec carried by 85/2082 and the type V SCCmec carried by WIS. The structures of these elements are illustrated based on the nucleotide sequence deposited in the DDBJ/EMBL/GenBank database under accession no. AB373032, AB037671, and AB121219. Conserved regions are indicated in blue: more than 99% homology is in dark blue and 19.1% to 93.6% homology is in light blue. Red arrowheads indicate integration site sequence of SCCmec (ISS) that comprises DR sequences, and black arrowheads indicate the positions and directions of primers used for detection of the novel type of SCCmec.