Objectives
The primary objective we sought to evaluate in this study was variation amongst lineages of MRSA, which have been successful in hospitals world-wide over decades. We also wished to explore genotypic differences that many explain potential variations in pathogenicity.

Methods
A composite virulence-associated gene microarray for the comparative analysis of clinical S. aureus strains was developed. The array comprises oligonucleotide probes for genes that have a potential virulence association. This microarray was used to investigate 61 MRSA strains, including representatives of ST1, 5, 8, 22, 30 and 239.

In addition, CA-MRSA (PVL⁺) and HA-MRSA (PVL^-) strains of the same MLST type were analysed to highlight patterns of genetic variability and provide insights into the enhanced virulence associated with CA-MRSA that may cause these effects. Although PVL expression is linked with the CA-MRSA phenotype, the elucidation of other differences may be necessary to link gene content and disease manifestation.

Results
Microarray results were called automatically using a Microsoft Excel spreadsheet and showed 70.2% (506) probes of the probes proved discriminatory, 29.796 % (212) were present in all strains and 0.04 % (3) were absent from all. Probe hybridisation was highly reproducible. Overall, analysis of the probe hybridisation data allowed a high level of discrimination between strains.

Conclusion
The microarray provides insights into the pathogenicity, epidemicity and evolution of S. aureus strains by providing profiles on their individual gene content and repertoire of virulence-associated genes. From an epidemiological perspective, this may enhance our ability to resolve variations between closely related strains.