A Subset of Staphylococcus aureus Strains Harboring Staphylococcal Cassette Chromosome mec (SCC mec ) Type IV Is Deficient in CcrAB-Mediated SCC mec Excision

Abstract

The gene encoding resistance to ␤-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 methicillinresistant 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.

Spontaneous Deletion of the Methicillin Resistance Determinant, mecA , Partially Compensates for the Fitness Cost Associated with High-Level Vancomycin Resistance in Staphylococcus aureus

“…The excision of SCCmec from the chromosome can be detected by PCR amplification across the chromosomal junction from which SCCmec was excised by using primers I1F and I1R, as described previously (34). By this approach, strain 5836VR yielded a positive amplification product, and DNA sequencing of this product revealed that the loss of mecA in this strain was due to site-specific SCCmec excision (data not shown).…”

“…CcrAB have been shown to catalyze the precise chromosomal excision of SCCmec, and so a likely explanation for this loss of DNA is that the SCCmec element was site-specifically excised from the chromosomes of strains 3130VR, VP32, and 5836VR (25,27,34). The excision of SCCmec from the chromosome can be detected by PCR amplification across the chromosomal junction from which SCCmec was excised by using primers I1F and I1R, as described previously (34).…”

“…To determine if CcrAB-mediated SCCmec excision is possible in strains 3130 and 5827, ccrAB was introduced into each strain on a multicopy plasmid, and the excision of SCCmec was monitored by PCR, as described previously (34). This method amplifies across the chromosomal junction from which SCCmec has excised as well as amplifies a fragment of the excised, circular SCCmec element.…”

“…Strain 450Mex is strain 450M with SCCmec site specifically excised from the chromosome. To accomplish this, plasmid-borne ccrAB was transduced into 450M (34). The transductants were passaged overnight, and an oxacillin-susceptible colony was isolated.…”

“…Plasmid-borne ccrAB was introduced into strains 3130 and 5827 by phage 80␣ transduction, and the strains were grown for 24 h in BHI broth prior to the isolation of total cellular DNA, as described previously (34). SCCmec excision was detected by a PCR-based approach.…”