on behalf of the PICR Study Group. Genomic epidemiology of meticillin-resistant Staphylococcus aureus ST22 widespread in communities of the Gaza StripBackground: Remarkably high carriage prevalence of a community-associated meticillin-resistant Staphylococcus aureus (MRSA) strain of sequence type (ST) 22 in the Gaza strip was reported in 2012. This strain is linked to the pandemic hospital-associated EMRSA-15. The origin and evolutionary history of ST22 in Gaza communities and the genomic

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... ts contributing to its widespread predominance are unknown. Methods: We generated high-quality draft genomes of 61 ST22 isolates from Gaza communities and, along with 175 ST22 genomes from global sources, reconstructed the ST22 phylogeny and examined genotypes unique to the Gaza isolates. Results: The Gaza isolates do not exhibit a close relationship with hospital-associated ST22 isolates, but rather with a basal population from which EMRSA-15 emerged. There were two separate resistance acquisitions by the same MSSA lineage, followed by diversification of other genetic determinants. Nearly all isolates in the two distinct clades, one characterised by staphylococcal cassette chromosome mec (SCCmec) IVa and the other by SCCmec V and MSSA isolates, contain the toxic shock syndrome toxin-1 gene. Discussion: The genomic diversity of Gaza ST22 isolates is not consistent with recent emergence in the region. The results indicate that two divergent Gaza clones evolved separately from susceptible isolates. Researchers should not assume that isolates identified as ST22 in the community are examples of EMRSA-15 that have escaped their healthcare roots. Future surveillance of MRSA is essential to the understanding of ST22 evolutionary dynamics and to aid efforts to slow the further spread of this lineage. 8