Gene Acquisition at the Insertion Site for SCC mec , the Genomic Island Conferring Methicillin Resistance in Staphylococcus aureus

Abstract

Staphylococcus aureus becomes resistant to methicillin by acquiring a genomic island, known as staphylococcal chromosome cassette mec (SCC mec ), which contains the methicillin resistance determinant, mecA . SCC mec is site-specifically integrated into the staphylococcal chromosome at a locus known as the SCC mec attachment site ( attB ). In an effort to gain a better understanding of the potential that methicillin-sensitive S. aureus (MSSA) isolates have for acquiring SCC mec , the nucleotide sequences of attB and surrounding DNA regions were examined in a diverse collection of 42 MSSA isolates. The chromosomal region surrounding attB varied among the isolates studied and appears to be a common insertion point for acquired foreign DNA. Insertions of up to 15.1 kb were found containing open reading frames with homology to enterotoxin genes, restriction-modification systems, transposases, and several sequences that have not been previously described in staphylococci. Two groups, containing eight and four isolates, had sequences found in known SCC mec elements, suggesting SCC mec elements may have evolved through repeated DNA insertions at this locus. In addition, the attB sequences of the majority of MSSA isolates in this collection differ from the attB sequences of strains for which integrase-mediated SCC mec insertion or excision has been demonstrated, suggesting that some S. aureus isolates may lack the ability to site-specifically integrate SCC me c into their chromosomes.