Objectives:

Surveillance of methicillin resistant Staphylococcus aureus (MRSA) in Canada began in 1995. From this surveillance, six epidemic strains of MRSA have been identified and named CMRSA1–6. In order to better understand the relatedness of these strains, as well as their genetic content, we have used microarrays to compare their genomes to that of the fully characterised genome of the MRSA strain Col.

Methods:

Genomic DNA from representatives of the six epidemic strains, as well as Col was fragmented and labelled using random primers with Cy5 or Cy3 labelled dCTP. Col and each of the epidemic strains (labelled with different dyes) were hybridised to arrays containing PCR products or 70 bp oligomers representing 2740 of the open reading frames (ORFs) in the Col genome. Data were processed with the ArrayPro software package, positive/negative cut-off values were determined using Genomotyping Analysis by Charles Kim and then analysed using the GeneMaths program. Macrorestriction digest patters were generated using SmaI.

Results:

Results indicate that all Canadian epidemic strains have six common regions of deletion, a portion of the type I SCCmec region, bacteriophage L54a, and four smaller areas composed of two to four ORFs. The only gene of known function in these smaller areas was the Staphylococcal enterotoxin B. Apart from these major deletions, many sporadic, single deletions are seen throughout the strains. Larger regions of deletion that are not present in all strains also occur. The only obvious ORF duplication is of IS431 in CMRSA3, 5 and 6, which is found in multiple copies in the typeIII SCCmec region in these strains. Macrorestriction digest data were used to approximate the sizes of the CMRSA genomes. CMRSA4 shows the smallest genome (~1862 kb), and the least genetic content in common with COL (79%). Though CMRSA1 and 2 appear to have larger genomes (~2378 and ~2706 kb, respectively), they show fewer ORFs in common with Col than other strains (81 and 86%, respectively), suggesting a substantial portion of the genome may be novel.

Conclusions:

This is the first study of epidemic MRSA using the comparative genomic hybridisation approach. While the CMRSA strains show a high degree of relatedness to Col, there are considerable differences in genetic content. This study also indicates that there may be genetic content which is unaccounted for in the Col genome. Other studies are being devised to identify and characterise the novel genetic content.