Salmonella enterica serovar Heidelberg (SH) is one of the prolific serovars causing poultry-associated food-borne illness in the world. Their ability to cause invasive infections and their promiscuity to plasmids that confer multidrug resistance to antibiotics of human health importance makes them a public health threat. Although, horizontal gene transfer (HGT) is recognized as the major mechanism used by Salmonella for acquiring antimicrobial resistance (AR) and virulence genes, the biology behind acquisition of new genes in SH is still unknown. In this study, we show that one day old broiler chicks challenged orally or via the cloaca with an antibiotic susceptible SH strain and raised without antibiotics carried susceptible and multidrug resistance SH strains 14 days after challenge. SH infection perturbed the bacterial community of broiler chicks and orally challenged chicks acquired AR at a higher rate than chicks challenged through the cloaca. Furthermore, SH strains lost and gained new genes, while some inverted their chromosome after colonizing the gut of broiler chicks. The acquisition of IncI1 plasmid multilocus sequence type 26 (pST26) from commensal Escherichia coli population present in the gut of broiler chicks conferred multidrug resistance phenotype to SH recipients and carriage of pST26 increased the fitness of SH under acidic selection pressure. Our results suggest that HGT shapes the evolution of AR in SH and that antibiotic use reduction alone is insufficient to limit AR plasmid transfer from commensal bacteria to Salmonella.

Host microbiome homeostasis ensures that gut conditions are unfavorable to an invading pathogen such as Salmonella enterica. Consequently, fostering a 'balanced' gut microbiome through the administration of microbes that can competitively exclude pathogens has gained a lot of attention and use in human and animal medicine. However, little is known on how competitive exclusion affects the transfer of antibiotic resistance. To shed more light on this question, we challenged neonatal broiler chicks raised on reused broiler chicken litter – a complex environment comprising of decomposing pine shavings, feces, uric acid, feathers, and feed, with Salmonella Heidelberg (S. Heidelberg), a model pathogen. We show that chicks raised on reused litter carried lower abundance of Salmonella and harbored a more uniform and diverse microbiome comprising of bacterial species that are known to provide colonization resistance towards Salmonella compared to chicks raised on fresh bedding composed of pine shavings. Additionally, these bacterial species were associated with a lower horizontal transfer of multidrug resistance genes to S. Heidelberg. Using in vitro competition experiments, we confirmed that conjugation between S. Heidelberg and E. coli strains from chicks raised on fresh litter resulted in the acquisition of multidrug resistant plasmids. Contrastingly, bacteriophage-mediated recombination between S. Heidelberg and E. coli strains made the acquisition of plasmid-mediated β-lactamase gene (blaCMY-2)possible. Collectively, this study demonstrates that competitive exclusion can reduce the transfer of antibiotic resistance and provides information on the bacterial species that can be explored for their benefits to reduce antibiotic resistance transfer.