Determination of Antimicrobial Resistance Megaplasmid-Like pESI Structures Contributing to the Spread of Salmonella Schwarzengrund in Japan

Background/Objectives: The acquisition of antimicrobial resistance by foodborne pathogens is a serious human health concern. In Japan, combinations of antimicrobial resistance genes in Salmonella from chicken meat were common among several serovars. Therefore, we hypothesized that different S. enterica serovars share a common antimicrobial resistance plasmid. Methods: Antimicrobial resistance transfer was tested in S. Infantis and S. Schwarzengrund, the major serovars used as donors. The plasmid structure was determined by subjecting S. Infantis Sal_238 and S. Schwarzengrund Sal_249 to short- and long-read sequencing. Results: The high homology between pSal_249Sch and pSal_238Inf suggests they have a common ancestor. Because the sequences of pSal_238Inf and pSal_249Sch were highly homologous to pESI (a plasmid for emerging S. Infantis), pSal_238Inf and pSal_249Sch were identified as pESI-like plasmids. S. Schwarzengrund is the third Salmonella serovar to expand its distribution related to pESI-like plasmid acquisition. Core-genome multilocus sequence-type analysis revealed that S. Schwarzengrund isolates with pESI-like plasmids from Japan (core-genome sequence-type [cgST] 167363 and cgST287831), the UK (cgST167363), and the USA (cgST167363, cgST196045, and cgST287831) were closely related; they are also suggested to share a common ancestor. The transfer of antimicrobial resistance was observed in combinations of both serovars. Specifically, the tentative plasmid sequence obtained via short-read sequencing, PCR, and conjugation experiments identified deletions of antimicrobial resistance genes (aadA, sul1, and tetA), class 1 integron, mercury resistance operon, and/or plasmid transfer region in the pESI-like plasmid. Conclusion: These data on the structural diversity of pESI-like plasmids suggest that some time has passed since S. Schwarzengrund acquired them.