High frequency and diversity of cassette chromosome recombinases (ccr) in methicillin-susceptible Staphylococcus...

Previous studies produced evidence that mecA, the determinant of β-lactam resistance in methicillin-resistant Staphylococcus aureus (MRSA), may have originated in the most primitive and widespread animal commensal species-Staphylococcus sciuri. But how the mecA homologue (mecA1/pbpD) was captured from S. sciuri into the staphylococcal cassette chromosome mec (SCCmec) has remained unclear.

To understand the role of S. sciuri in the assembly of SCCmec, we screened 118 methicillin-susceptible S. sciuri isolates for SCCmec central elements-ccr and mec complex (ccrAB, ccrC, mecA, mecI and mecR1)-by dot-blot hybridization. In addition, isolates were typed by PFGE and the chromosomal proximity of SCCmec elements was determined by Southern hybridization. ccr typing was performed by nucleotide sequencing.

ccrAB were identified in 35% of the isolates (n = 41), represented by 24 PFGE types, but ccrC was not found. None of the isolates carried mecA or its regulators, but all isolates carried mecA1/pbpD. In the majority of isolates, ccr and mecA1 were located near orfX, the SCCmec integration site. Moreover, in 31% (n = 13) of the ccrAB-carrying strains, ccrAB, mecA1 and orfX colocalized in the chromosome. The nucleotide sequence of ccrA/ccrB was highly diverse, including ccr genes closely related (80%-97%) to those found in MRSA.

Our results suggest that S. sciuri was a natural recipient and a rich reservoir of ccr for the assembly of SCCmec. The chromosomal location of mecA1, near orfX, the recognition site of ccr, was probably crucial for its mobilization out of S. sciuri species into SCCmec.