Antimicrobial Resistance of Salmonella and Characterization of Two Mcr-1-Harboring Isolates from Pork Products in Guangdong, China

<i>Salmonella</i> is a major foodborne pathogen that poses an increasing threat due to the emergence of <i>mcr-1</i>-mediated colistin resistance. However, data on <i>mcr-1</i>-positive <i>Salmonella</i> in pork products are limited. In this study, 457 samples collected in 2023 from pig slaughterhouses in Guangdong province were investigated to determine the prevalence and genomic characteristics of <i>mcr-1</i>-positive <i>Salmonella</i> isolates. We found that 92 <i>Salmonella</i> isolates (20.1%, 92/457) were recovered, representing six serotypes, including <i>Salmonella Typhimurium</i> (<i>n</i> = 29) and <i>Salmonella</i> Rissen (<i>n</i> = 29). High resistance to tetracycline (90.2%, 83/92) and multidrug resistance (58.7%, 54/92) were observed. Critically, two colistin-resistant <i>Salmonella Kentucky</i> ST198 isolates (2.2%) harboring <i>mcr-1</i> on transferable IncI2 plasmids were identified. Genomic analysis revealed a novel multidrug resistance region (MRR, ~57 kb) inserted into the <i>bcfH</i> locus (containing <i>floR</i>, <i>qnrS1</i>, <i>bla</i>_CTX-M-55, and <i>aph (3')-Ia</i>) and a variant <i>Salmonella</i> Genomic Island 1 (SGI1-KI, containing <i>tet (A)</i>, <i>sul1</i>, <i>qacEΔ1</i> and <i>aadA7</i>) in these isolates. The MRR and SGI1-KI may enhance bacterial survival under antibiotic selection pressure. Phylogenetic analysis showed close relatedness to human clinical strains, suggesting food chain transmission. The findings highlight a high antimicrobial resistance burden, the emergence of transferable last-resort colistin resistance (<i>mcr-1</i>), and acquisition of complex resistance determinants (MRR, SGI1-KI), underscoring an urgent need for enhanced "One Health" surveillance.