Antimicrobial Resistance Genes and Clonal Relationships of Duck-Derived Salmonella in Shandong Province, China in 2023

<i>Salmonella</i> is a major threat to both human and animal health. However, the diversity and antibiotic resistance of animal-derived <i>Salmonella</i> and their association with human infections remain largely unexplored. In this study, <i>Salmonella</i> strains were isolated, identified, and sequenced from dead embryos and cloacal swab samples obtained from 278 large-scale duck farms in 11 cities in Shandong Province. The results show that a total of 57 <i>Salmonella</i> strains were isolated, with the dominant sequence types (ST) being ST17 (15/57) and ST19 (9/57), while the dominant serotypes were <i>S. Indiana</i> (15/57) and <i>S. Typhimurium</i> (11/57). Furthermore, genomic analysis has revealed the presence of prevalent antibiotic resistance genes (ARGs), which are often associated with co-transfer mechanisms. Over 52.63% of the strains were observed to carry two or more ARGs, especially one <i>Salmonella</i> strain that carried twenty-eight distinct ARGs. Furthermore, core genome multilocus sequence typing analysis (cgMLST) indicated that the 57 <i>Salmonella</i> strains may have a close relationship, which could be clonally transmitted among different cities. The results demonstrated a close relationship between the <i>Salmonella</i> strains identified in diverse geographical regions, suggesting that these strains may have been widely disseminated through clonal transmission. The mutation analysis reveals significant mutations at <i>parC</i> (T57S), <i>gyrA</i> (S83F), <i>parC</i> (S80R), <i>gyrA</i> (D87N), and <i>gyrA</i> (S83Y). These findings emphasize the necessity for monitoring and controlling <i>Salmonella</i> infections in animals, as they may serve as a reservoir for ARGs with the potential to affect human health or even be the source of pathogens that infect humans.