Antimicrobial resistance and plasmid-associated virulence genes in<i> Salmonella</i> isolated from pigs, pork, and humans in border provinces of Thailand and neighboring countries

In Southeast Asia, most antimicrobial resistance (AMR) data on <i>Salmonella</i> have been generated at the phenotypic level, while insights into the genetic characteristics of AMR and virulence genes remain limited. This study aimed to further characterize AMR <i>Salmonella</i> isolates carrying plasmid-associated virulence genes in Thailand and neighboring countries. A total of 366 <i>Salmonella</i> isolates were collected from pigs (<i>n</i> = 265), pork (<i>n</i> = 69), and humans (<i>n</i> = 32) across Thailand, Lao People's Democratic Republic, Cambodia, and Myanmar. Overall, 69.40% (<i>n</i> = 254/366) were multidrug resistant, including resistance to medically important antimicrobials tigecycline, azithromycin, colistin and ciprofloxacin. Whole genome sequencing analysis revealed that five <i>Salmonella</i> Enteritidis ST11 clinical isolates from different patients in different provinces carried IncFIB/IncFII plasmids with plasmid-associated virulence genes (<i>spvBCD, pefABCD</i>, <i>rck</i>, and <i>mig-5</i>), of which three of them (SA615, SA616 and SA617) additionally harbored IncX1 plasmid carrying <i>bla</i> _TEM-135. Virulence plasmids (pSEVTs) exhibited a close relationship with the pSLT of <i>Salmonella</i> Typhimurium LT2 except for two absent segments (PSLT056-PSLT057-PSLT059 PSLT060-PSLT061-PSLT062-PSLT063-064-ssB-PSLT067) and (<i>traR-traC-trabI-traW-traU trbC-traN-trbE-traF-traQ-traQ-trbB-traH-traD-traH-traI-traX</i>) as well as two invert regions, R1 (locus tag PSLT001 to <i>repA2</i>) and R2 (PSLT025 to <i>finO</i> with deleted <i>tra</i>) in pSEVTs. None of the plasmids were horizontally transferred under ampicillin selective pressure. Phylogenetic analysis of whole genome sequence and virulence plasmids revealed the clonal dissemination of the isolates. The co-existence of virulence and resistance plasmids may complicate antibiotic therapy, highlighting the need to monitor plasmid-associated virulence genes alongside AMR genes in surveillance programs for humans and animals.