Novel Staphylococcal Cassette Chromosome mec Type, Tentatively Designated Type VIII, Harboring Class A mec and Type 4 ccr Gene Complexes in a Canadian Epidemic Strain of Methicillin-Resistant Staphylococcus aureus

Abstract

Staphylococcal cassette chromosome mec (SCC mec ) is a mobile genetic element characterized by flanking terminal direct and, in most cases, inverted repeat sequences, the mec and ccr gene complexes, and their surrounding DNA regions. Unique combinations of the mec and ccr gene complexes generate various SCC mec types. Six SCC mec types have been reported to date. We describe here a novel SCC mec type identified in a Canadian methicillin-resistant Staphylococcus aureus (MRSA) epidemic strain. MRSA clinical isolates were screened for known SCC mec types by multiplex and conventional PCR methods. Three phenotypically and genotypically identical MRSA clinical isolates with a pulsotype identical to CMRSA9 were identified locally and found to be nontypeable by available SCC mec typing schemes. Complete sequencing of the SCC mec element revealed a nucleotide fragment of 32,168 bp integrated at an identical chromosomal integration site ( att B scc ) at the 3′ end of the orfX gene. The nucleotide sequences at the chromosome-SCC mec junction regions were typical of other SCC mec types, but the element harbored a unique combination of class A mec and type 4 ccr gene complexes. Sequence recombination analysis suggested that this unique SCC mec type may be derived from homologous recombination between the previously described SCC _RP62A of S. epidermidis strain RP62A and SCC composite island of S. epidermidis ATCC 12228, respectively, or via recombination of other staphylococcal strains that carry the same or similar mobile cassettes. We identified a previously undescribed type of SCC mec from isolate C10682, tentatively designated type VIII, and we provide compelling evidence supporting the ability of SCC elements to transfer horizontally or undergo recombination to generate new SCC mec types.