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

In Southeast Asia, most antimicrobial resistance (AMR) data on Salmonella 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 Salmonella isolates carrying plasmid-associated virulence genes in Thailand and neighboring countries. A total of 366 Salmonella isolates were collected from pigs ( n = 265), pork ( n = 69), and humans ( n = 32) across Thailand, Lao People’s Democratic Republic, Cambodia, and Myanmar. Overall, 69.40% ( n = 254/366) were multidrug resistant, including resistance to medically important antimicrobials tigecycline, azithromycin, colistin and ciprofloxacin. Whole genome sequencing analysis revealed that five Salmonella Enteritidis ST11 clinical isolates from different patients in different provinces carried IncFIB/IncFII plasmids with plasmid-associated virulence genes ( spvBCD, pefABCD , rck , and mig-5 ), of which three of them (SA615, SA616 and SA617) additionally harbored IncX1 plasmid carrying bla TEM-135 . Virulence plasmids (pSEVTs) exhibited a close relationship with the pSLT of Salmonella Typhimurium LT2 except for two absent segments (PSLT056-PSLT057-PSLT059 PSLT060-PSLT061-PSLT062-PSLT063-064-ssB-PSLT067) and ( traR-traC-trabI-traW-traU trbC-traN-trbE-traF-traQ-traQ-trbB-traH-traD-traH-traI-traX ) as well as two invert regions, R1 (locus tag PSLT001 to repA2 ) and R2 (PSLT025 to finO with deleted tra ) 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.