Between 2003 and 2008, seventy-six clinical isolates of a Panton-Valentine leucocidin (PVL)-positive ST8-MRSA-IV strain were isolated from seventy-two non hospitalised patients living in Western Australia. Colloquially known as WA MRSA-12 All isolates were spa type t008 and had a USA300-like pulsed field gel electrophoresis pulsotype. Approximately 95% of isolates were recovered from skin and soft tissue infections.

Genotyping was performed on all isolates using a diagnostic DNA microarray which included species markers, resistance factors, virulence-associated determinants, and MSCRAMM genes. Strains varied in their carriage of the blaZ/I/R, ermC, msrA and mpbBM, aadD and mupR, aphA3 and sat, tetK, qacC, merA/B/R/T resistance genes, the beta-haemolysin-converting phages, and the enterotoxin (sek and seq) genes. The arginine catabolic mobile element (ACME) was absent in 15.8% of isolates. The mercury resistance (mer) operon, which is usually associated with SCCmec type III elements was found in several ACME-negative isolates.

Hybridisation profiles identified sixteen variants or subtypes. 47 (61.8%) strains were identified as “Variant A” which was identical to the sequenced USA300-TC1516 strain. The earliest isolate of USA300 detected in Western Australia (July 2003) belonged to this subtype suggesting other variants may have evolved from “Variant A” as a result of a limited number of gene losses or acquisitions. Investigations on three family outbreaks showed that the analysis of intrastrain variability by DNA microarray may be helpful for tracing transmission.

This study emphasizes the importance of molecular typing in detecting the introduction and evolution of significant MRSA strains within a community. DNA microarray technology has been useful in characterising the original isolate of WA MRSA 12 as USA300-TC1516 and identifying the transmission of variants that have been emerged over time.