Assessment of allelic variation in the ccrAB locus in methicillin-resistant Staphylococcus aureus clones

Objectives: To evaluate the allelic variability of ccrA and ccrB among methicillin-resistant Staphylococcus aureus (MRSA) strains belonging to different genetic lineages and carrying different SCCmec types. Methods: Internal fragments from both genes were obtained by PCR amplification with pairs of degenerate primers and the nucleotide sequence was determined for both strands. Nucleotide sequences were aligned and the phylogenetic trees were reconstructed. Results: The homology scores obtained confirmed the very close relationships among ccrAB alleles associated with each SCCmec type, which further validate this locus as a good target for SCCmec typing strategies. Using this strategy, particularly for the ccrB gene, SCCmec types with the same ccrAB allotype, as detected by PCR, could be discriminated and some strains with non-typeable ccrAB loci were resolved. Conclusions: We propose that sequencing an internal fragment of ccrB is a strategy for SCCmec typing that may be easily incorporated into other sequence-based MRSA typing strategies, such as multi-locus sequence typing and spa typing.