Determination of Antimicrobial Resistance Megaplasmid-Like pESI Structures Contributing to the Spread of 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 from chicken meat were common among several serovars. Therefore, we hypothesized that different serovars share a common antimicrobial resistance plasmid.

Methods: Antimicrobial resistance transfer was tested in . Infantis and . Schwarzengrund, the major serovars used as donors. The plasmid structure was determined by subjecting . Infantis Sal_238 and . 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 Infantis), pSal_238Inf and pSal_249Sch were identified as pESI-like plasmids. . Schwarzengrund is the third serovar to expand its distribution related to pESI-like plasmid acquisition. Core-genome multilocus sequence-type analysis revealed that . 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 (, , and ), 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 . Schwarzengrund acquired them.