A subset of Staphylococcus aureus strains harboring staphylococcal cassette chromosome mec (SCCmec) type IV is deficient in CcrAB-mediated SCCmec excision.

The gene encoding resistance to beta-lactam antibiotics in the staphylococci is found on the chromosome in a genomic island designated staphylococcal cassette chromosome mec, or SCCmec. In addition to the resistance gene mecA, SCCmec also contains site-specific recombinase genes that are capable of catalyzing the chromosomal excision and reintegration of SCCmec. SCCmec is found in five major isotypes partially defined by the recombinase genes present, either ccrAB or ccrC. Of these, SCCmec type IV is presumed to be mobile in the environment, and this mobility may be partially responsible for the rise in community-associated methicillin-resistant staphylococcal infections. In this study, we investigate the presumptive first step in type IV SCCmec mobility: chromosomal excision of the element. CcrAB from a panel of six Staphylococcus aureus and four Staphylococcus epidermidis strains were able to catalyze chromosomal excision of SCCmec types I and II, indicating that these proteins maintain recombinase activity despite varying by up to 3.7% at the amino acid level. Excision of type IV SCCmec was not universally seen, as a subset of S. aureus strains with type IV SCCmec did not excise their element. These strains are all highly related and represent a lineage of successful community-associated pathogens. In addition, the inability to excise SCCmec in these strains is associated with the insertion of a presumptive mobile element containing the gene for staphylococcal enterotoxin H (seh) immediately downstream of SCCmec on the chromosome. Acquisition of this mobile element, containing a known virulence gene, appears to have stabilized the chromosomal integration of the methicillin resistance gene in these strains.