Novel mobile variants of staphylococcal cassette chromosome mec in Staphylococcus aureus

Staphylococcus aureus staphylococcal cassette chromosome mec type IV (SSCmec IV) is associated with virulent community-acquired methicillin-resistant Staphylococcus aureus (MRSA) and frequent horizontal transfer among staphylococci. To gain insight into the mechanism of transfer, we studied the ccrAlB type 2 recombinase-mediated excision of SCCmec IV (n = 5 strains) and SCCmec II (n = 2). In SCCmec IV- but not SCCmec II-containing strains, spontaneous excision of the cassette was observed. Introduction of ccrAlB type 2 recombinase genes under control of an S. aureus bacterial phage promoter in the different strains yielded excision of SCCmec II and multiple excision variants of SCCmec IV. Sequencing of the alternatively excised products in SCCmec IV strains identified a 100-bp shortened SCCmec' variant and a 5,877-bp, conserved SCC-like element that lacks mecA and ccrA/B recombinases. Excision of the SCC-like element in wild-type S. aureus was dependent on the presence of SCCmec. The element could be excised separately or as part of a novel composite cassette together with SCCmec. The relative abundance of and variety in SCCmec IV excisions may contribute to the frequency of horizontal transfer and genetic plasticity in SCCmec IV MRSA strains.