Whole-genome sequencing and phylogenetic analysis capture the emergence of a multi-drug resistant Salmonella enterica serovar Infantis clone from diagnostic animal samples in the United States

INTRODUCTION: Salmonella enterica is a major cause of foodborne illness in the United States. A multi-drug resistant (MDR) emergent Salmonella Infantis (ESI) with a megaplasmid (pESI) was first identified in Israel and Italy and subsequently reported worldwide. The ESI clone carrying an extended spectrum β-lactamase blaCTX-M-65 on a pESI-like plasmid and a mutation in the gyrA gene has recently been found in the United States in poultry meat. METHODS: We analyzed the phenotypic and genotypic antimicrobial resistance, genomics and phylogeny of 200 S. infantis isolates from animal diagnostic samples. RESULTS: Of these, 33.5% were resistant to at least one antimicrobial and 19.5% were multi-drug resistant (MDR). Eleven isolates from different animal sources were phenotypically and genetically similar to the ESI clone. These isolates had a D87Y mutation in the gyrA gene conferring reduced susceptibility to ciprofloxacin and harbored a combination of 6–10 resistance genes: blaCTX-M-65, aac(3)-IVa, aadA1, aph(4)-Ia, aph(3′)-Ia, floR, sul1, dfrA14, tetA, and fosA. These 11 isolates carried class I and class II integrons and three virulence genes: sinH, involved in adhesion and invasion, ybtQ and ybtP, associated with iron transport. These isolates were also closely related to each other (separated by 7 to 27 SNPs) and phylogenetically related to the ESI clone recently found in the U.S. DISCUSSION: This dataset captured the emergence of the MDR ESI clone in multiple animal species and the first report of a pESI-like plasmid in isolates from horses in the U.S.