| Summary: | <i>Salmonella</i> Infantis carrying extended spectrum β-lactamase <i>bla</i><sub>CTX-M-65</sub> on a pESI-like megaplasmid has recently emerged in United States poultry. In order to determine the carriage rate and gene content variability of this plasmid in U.S. <i>Salmonella</i> Infantis, whole genome sequences of <i>Salmonella</i> isolates from humans and animals in the U.S. and internationally containing the pESI-like plasmid were analyzed. The U.S. Department of Agriculture Food Safety and Inspection Service (FSIS) identified 654 product sampling isolates containing pESI-like plasmids through hazard analysis and critical control point (HACCP) verification testing in 2017 and 2018. The Centers for Disease Control and Prevention identified 55 isolates with pESI-like plasmids in 2016–2018 through the National Antimicrobial Resistance Monitoring System. Approximately 49% of pESI-like plasmids from FSIS verification isolates and 71% from CDC NARMS contained <i>bla</i><sub>CTX-M-65</sub>. Pan-plasmid genome analysis was also performed. All plasmids contained <i>traN</i> and more than 95% contained 172 other conserved genes; 61% contained <i>bla</i><sub>CTX-M-65</sub>. In a hierarchical clustering analysis, some plasmids from U.S. animal sources clustered together and some plasmids from South America clustered together, possibly indicating multiple plasmid lineages. However, most plasmids contained similar genes regardless of origin. Carriage of the pESI-like plasmid in U.S. appears to be limited to <i>Salmonella</i> Infantis and carriage rates increased from 2017 to 2018.
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