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<jats:title>ABSTRACT</jats:title> <jats:p> <jats:italic>Staphylococcus argenteus</jats:italic> is a newly named species previously described as a divergent lineage of <jats:italic>Staphylococcus aureus</jats:italic> that has recently been shown to have a global distribution. Despite growing evidence of the clinical importance of this species, knowledge about its population epidemiology and genomic architecture is limited. We used whole-genome sequencing to evaluate and compare <jats:italic>S. aureus</jats:italic> ( <jats:italic>n</jats:italic> = 251) and <jats:italic>S. argenteus</jats:italic> ( <jats:italic>n</jats:italic> = 68) isolates from adults with staphylococcal sepsis at several hospitals in northeastern Thailand between 2006 and 2013. The majority (82%) of the <jats:italic>S. argenteus</jats:italic> isolates were of multilocus sequence type 2250 (ST2250). <jats:italic>S. aureus</jats:italic> was more diverse, although 43% of the isolates belonged to ST121. Bayesian analysis suggested an <jats:italic>S. argenteus</jats:italic> ST2250 substitution rate of 4.66 (95% confidence interval [CI], 3.12 to 6.38) mutations per genome per year, which was comparable to the <jats:italic>S. aureus</jats:italic> ST121 substitution rate of 4.07 (95% CI, 2.61 to 5.55). <jats:italic>S. argenteus</jats:italic> ST2250 emerged in Thailand an estimated 15 years ago, which contrasts with the <jats:italic>S. aureus</jats:italic> ST1, ST88, and ST121 clades that emerged around 100 to 150 years ago. Comparison of <jats:italic>S. argenteus</jats:italic> ST2250 genomes from Thailand and a global collection indicated a single introduction into Thailand, followed by transmission to local and more distant countries in Southeast Asia and further afield. <jats:italic>S. argenteus</jats:italic> and <jats:italic>S. aureus</jats:italic> shared around half of their core gene repertoire, indicating a high level of divergence and providing strong support for their classification as separate species. Several gene clusters were present in ST2250 isolates but absent from the other <jats:italic>S. argenteus</jats:italic> and <jats:italic>S. aureus</jats:italic> study isolates. These included multiple exotoxins and antibiotic resistance genes that have been linked previously with livestock-associated <jats:italic>S. aureus</jats:italic> , consistent with a livestock reservoir for <jats:italic>S. argenteus</jats:italic> . These genes appeared to be associated with plasmids and mobile genetic elements and may have contributed to the biological success of ST2250. </jats:p> <jats:p> <jats:bold>IMPORTANCE</jats:bold> In this study, we used whole-genome sequencing to understand the genome evolution and population structure of a systematic collection of ST2250 <jats:italic>S. argenteus</jats:italic> isolates. A newly identified ancestral species of <jats:italic>S. aureus</jats:italic> , <jats:italic>S. argenteus</jats:italic> has become increasingly known as a clinically important species that has been reported recently across various countries. Our results indicate that <jats:italic>S. argenteus</jats:italic> has spread at a relatively rapid pace over the past 2 decades across northeastern Thailand and acquired multiple exotoxin and antibiotic resistance genes that have been linked previously with livestock-associated <jats:italic>S. aureus</jats:italic> . Our findings highlight the clinical importance and potential pathogenicity of <jats:italic>S. argenteus</jats:italic> as a recently emerging pathogen. </jats:p>

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American Society for Microbiology

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