High prevalence of multidrug-resistant <i>Salmonella enterica</i> in Thailand food markets: insights from complete genome and phenotypic characterization of ESBL-producing strains

<i>Salmonella</i> contamination in the food chain is a common root of foodborne outbreaks. The transmission of antimicrobial resistance among <i>Salmonella</i> and food bacteria raises global public health concern. As a major food exporter and travel hub in Southeast Asia, Thailand is particularly susceptible. To better understand the circulating <i>Salmonella</i>, we characterized 74 non-typhoidal <i>Salmonella</i> isolates previously obtained from Thailand food markets. Whole genome sequencing, drug susceptibility testing, and host invasion efficiency assays were performed. The most prevalent serotype was <i>Salmonella</i> serotype Rissen (32.43%) followed by Derby (12.16%). Fifty-two isolates were multidrug-resistant (MDR), with their primary resistance pattern being ampicillin, streptomycin, and tetracycline resistance. The most resistant strain was <i>S</i>. Rissen exhibiting resistance to 13 tested drugs, including third-generation cephalosporin. Six cephalosporin-resistant isolates were identified, four of which were extended-spectrum β-lactamase (ESBL)-producing strains harboring the <i>bla</i> _CTX-M-14 or <i>bla</i> _CTX-M-55 ESBL gene. Complete genomes showed that three ESBL-producing isolates contained plasmid-mediated ESBL, and the other was chromosomal-mediated. <i>In vitro</i> invasion efficiencies of ESBL-producing strains into mucous-producing human colorectal adenocarcinoma HT29-MTX-E12 cells were compared to the reference strain <i>Salmonella</i> Typhimurium LT2. Statistically significant higher invasion efficiency in the chromosomal-mediated ESBL strain was found. Interestingly, the ESBL plasmid detected during this study was highly similar to a plasmid previously isolated from a Thai patient. This study highlights that the majority of <i>Salmonella</i> isolated from Thai foods are MDR and potentially ESBL-producing strains, highlighting the need for continued surveillance and intervention strategies.IMPORTANCEIn Thailand, fermented foods are typically consumed raw, and pork is considered a delicacy of Thai cuisine. The presence of multidrug resistant (MDR) foodborne pathogens in these food types raises concern and presents a risk to public health. Here, we report that <i>Salmonella</i> Rissen was the most prevalent serotype isolated from these food samples in Thailand. All isolates carried virulence gene clusters crucial for pathogenesis, and more than 70% of isolates were MDR strains. Four of the MDR strains were ESBL-producing. Whole genome sequence analysis and phenotypic characterizations revealed that chromosome-mediated ESBL strains possessed higher <i>in vitro</i> invasion efficiency than plasmid-mediated ESBL strains. This study highlights two key public health threats: the risk of acquiring difficult-to-treat MDR <i>Salmonella</i> infections from undercooked food and the circulation of AMR plasmids in fresh markets in Thailand.