Multidrug resistance in Salmonella isolates of swine origin: mobile genetic elements and plasmids associated with cephalosporin resistance with potential transmission to humans

ABSTRACT The emergence of foodborne Salmonella strains carrying antimicrobial resistance (AMR) in mobile genetic elements (MGE) is a significant public health threat in a One Health context requiring continuous surveillance. Resistance to ciprofloxacin and cephalosporins is of particular concern. Since pigs are a relevant source of foodborne Salmonella for human beings, we studied transmissible AMR genes and MGE in a collection of 83 strains showing 9 different serovars and 15 patterns of multidrug resistant (MDR) previously isolated from pigs raised in the conventional breeding system of Northern Spain. All isolates were susceptible to ciprofloxacin and three isolates carried bla CMY-2 or bla CTX-M-9 genes responsible for cefotaxime resistance. Filter mating experiments showed that the two plasmids carrying bla CTX-M-9 were conjugative while that carrying bla CMY-2 was self-transmissible by transformation. Whole-genome sequencing and comparative analyses were performed on the isolates and plasmids. The IncC plasmid pSB109, carrying bla CMY-2 , was similar to one found in S . Reading from cattle, indicating potential horizontal transfer between serovars and animal sources. The IncHI2 plasmids pSH102 in S . Heidelberg and pSTM45 in S . Typhimurium ST34, carrying bla CTX-M-9 , shared similar backbones and two novel “complex class 1 integrons” containing different AMR and heavy metal genes. Our findings emphasize the importance of sequencing techniques to identify emerging AMR regions in conjugative and stable plasmids from livestock production. The presence of MGE carrying clinically relevant AMR genes raises public health concerns, requiring monitoring to mitigate the emergence of bacteria carrying AMR genes and subsequent spread through animals and food. IMPORTANCE The emergence of foodborne Salmonella strains carrying antimicrobial resistance (AMR) in mobile genetic elements (MGE) is a significant public health threat in a One Health context. Since pigs are a relevant source of foodborne Salmonella for humans, in this study, we investigate different aspects of AMR in a collection of 83 Salmonella showing nine different serovars and 15 patterns of multidrug resistant (MDR) isolated from pigs raised in the conventional breeding system. Our findings emphasize the importance of sequencing techniques to identify emerging AMR regions in conjugative and stable plasmids from livestock production. The presence of MGE carrying clinically relevant AMR genes raises public health concerns, requiring monitoring to mitigate the emergence of bacteria carrying AMR genes and subsequent spread through animals and food.