OBJECTIVE: SCCmec is a highly mobile genetic element characterized by flanking direct/inverted repeat sequences, the mec- and ccr-gene complexes, and their surrounding DNA regions, unique combinations of which generate various SCCmec types. Six SCCmec types have been reported to date. We describe a novel SCCmec type identified in a Canadian MRSA (CMRSA) epidemic strain.

METHODS: MRSA clinical isolates were screened for known SCCmec types by multiplex/conventional PCR methods, and further characterized by PFGE/MLST/spa/agr typing. The DNA fragment encompassing the SCCmec element of one unique non-typeable strain was amplified by long-range PCR, with nucleotide sequence determination by primer-extension. Sequence analysis was performed using various bioinformatic programs.

RESULTS: Three phenotypically and genotypically identical MRSA clinical isolates with a PFGE pattern identical to CMRSA9 (MLST-ST8; spa-t008; agr-I) were identified locally and found to be nontypeable by available SCCmec typing schemes. Complete sequencing of the SCCmec element revealed a nucleotide fragment size of 32-kbs, an identical chromosomal integration site (attBscc) within orfX, and similar characteristic nucleotide sequences at the chromosome-SCCmec junction regions typical of other SCCmec types, but harbouring a unique recombination of Class A mec and Type 4 ccr gene complexes. Sequence recombination analysis suggested that this unique SCCmec type was derived from homologous recombination between previously described SCC-RP62a and SCCcomposite-island of S. epidermidis strains RP62a and ATCC12228, respectively.

CONCLUSIONS: We identified a previously undescribed type of SCCmec (tentatively designated type VIII) and provide compelling evidence supporting the ability of SCC elements to transfer horizontally or undergo recombination to generate new SCCmec types.