Genomic characterisation of CC398 MRSA causing severe disease in Australia.

Clonal complex 398 (CC398) livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) has been reported worldwide in a variety of food-animal species. Although CC398 is synonymous with LA-MRSA, community-associated MRSA (CA-MRSA) variants have emerged, including the Panton-Valentine leukocidin (PVL)-positive ST398-V and ST398 single-locus variant ST1232-V, and the PVL-negative ST398-V clones. Using comparative genomic analysis, we determined whether ten CC398 MRSA bacteraemia episodes recently identified in Australia were due to LA-MRSA or CA-MRSA CC398. Isolates were sourced from the Australian Group on Antimicrobial Resistance S. aureus surveillance programme and episodes occurred across Australia. Whole-genome sequencing (WGS) and phylogenetic comparison of the ten CC398 bacteraemia isolates with previously published CC398 MRSA whole-genome sequences identified that the Australian CC398 isolates were closely related to the human-associated II-GOI clade and the livestock-associated IIa clade. The identified CC398 MRSA clones were: PVL-positive ST1232-V (5C2&5), PVL-negative community-associated ST398-V (5C2&5) and livestock-associated ST398-V (5C2&5). Our findings demonstrate the importance of using WGS and comparing the sequences with international sequences to distinguish between CC398 CA-MRSA and LA-MRSA and to determine the isolates' origin. Furthermore, our findings suggest that CC398 CA-MRSA has become established in the Australian community and that ST398-V (5C2&5) LA-MRSA is now widespread in Australian piggeries. Our study emphasises the need for national One Health antimicrobial resistance surveillance programmes to assist in monitoring the ongoing epidemiology of MRSA and other clinically significant antimicrobial-resistant organisms.