Molecular Epidemiology of Salmonella Isolated from Pigs Reared in Distinct Swine Production Systems and Humans

Salmonella is one of the most important foodborne pathogens causing salmonellosis in both humans and animals. Emergence and persistence of antimicrobial resistant (AMR) foodborne pathogens due to routine use of antimicrobials for therapeutic, preventive and growth purposes in conventional food animal production and its association with the development of AMR bacterial strains is a growing public health concern. Therefore, increasing consumer preference of antimicrobial-free (ABF) meat products makes it important to identify risk factors associated with the transmission of AMR foodborne pathogens in these unique production systems.;The objectives of this study were to 1) determine the dynamics of Salmonella in pigs reared in distinct production systems, 2) characterize and compare isolates from pigs, the environment and humans at the phenotypic and genotypic level, and 3) identify conjugative plasmids and determine their inter- serovar exchange among Salmonella of human, pig and environmental origin. To address the above objectives, we collected pig and environmental samples from 10 conventional and eight ABF longitudinal cohorts at different stages of production and slaughter. A total of 1,090 Salmonella isolates was recovered with a significantly higher prevalence in conventionally reared pigs (4%) and their environment (12%) compared to ABF pigs (0.2%) and their environment (0.6%) (P<0.001). There were 24 different serotypes identified with Salmonella Typhimurium, Salmonella Anatum, Salmonella Infantis, Salmonella Rissen and Salmonella Derby being predominant. At the same period of time we collected 572 human clinical Salmonella isolates from North Carolina state laboratory of public health.;Antimicrobial susceptibility testing revealed that conventional isolates exhibited higher AMR than human clinical and ABF isolates. Overall, the highest frequency of resistance in Salmonella isolates was against TET, followed by FIS and STR with the exception of AMI and CIP resistance which were only exhibited by human clinical isolates. We also detected resistance to 3rd and 4th generation of cephalosporins by Salmonella isolates of three different sources. Conventional isolates also had a higher frequency of multidrug resistant (MDR) isolates compared to human and ABF isolates. We detected multiple AMR genes including blaCMY-2, blaTEM, blaPSE, cmlA, strA/B, aadA1/A2, aphAI, tet(A),(B),(C) and (G) along with class I and II integrons among Salmonella isolates.;Genotyping of temporally and spatially related Salmonella isolates from human clinical cases, pigs and their environment from two production systems by Pulsed field gel electrophoresis (PFGE) analysis revealed a similar fingerprint profile among the S. Typhimurium serotype, which is commonly associated foodborne outbreaks, among all three sources. Detection of 100% genotypic similarity among serotypes isolated from pigs and their environment belonging to the same cohort at different stages of production at farm and slaughter potentially explains the dissemination of Salmonella throughout the production chain.;The potential development and propagation of MDR in Salmonella through mobile genetic elements was further evidenced by identification of conjugal plasmids in MDR Salmonella isolates. Characterization of these plasmids with restriction enzymes and southern hybridization provided clear evidence of the presence of similar plasmids circulating among MDR Salmonella isolates of human, pig and environmental origin.;In conclusion, our study results demonstrate the presence of AMR Salmonella in ABF and conventional production systems at farm, slaughter and the environment. The phenotypic and genotypic profiles underscore the potential role played by the environment in the persistence and dissemination of AMR Salmonella in the two production systems and in humans. Dissemination of identical conjugal plasmids carrying multiple resistance determinants and its inter-serovar irrespective of source of origin is essential to understand the spread and persistence of antimicrobial resistance in the food chain. Further studies needed to determine the role played by the environment in occurrence and dissemination of AMR Salmonella in the food chain.