Tetracycline resistance gene transfer from Escherichia coli donors to Salmonella Heidelberg in chickens is impacted by the genetic context of donors

Authors: GUERNIER-CAMBERT, VANINA - Oak Ridge Institute For Science And Education (ORISE); Trachsel, Julian; Atkinson, Briony; Oladeinde, Adelumola; Anderson, Christopher; Bearson, Shawn; Monson, Melissa; Looft, Torey

Submitted to: Veterinary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2024
Publication Date: 11/4/2024
Citation: Guernier-Cambert, V., Trachsel, J.M., Atkinson, B.M., Oladeinde, A.A., Anderson, C.L., Bearson, S.M., Monson, M.S., Looft, T.P. 2024. Tetracycline resistance gene transfer from Escherichia coli donors to Salmonella Heidelberg in chickens is impacted by the genetic context of donors. Veterinary Microbiology. 299. Article 110294. https://doi.org/10.1016/j.vetmic.2024.110294.
DOI: https://doi.org/10.1016/j.vetmic.2024.110294

Interpretive Summary: The spread of Salmonella to humans through contaminated food, especially undercooked poultry, is a leading cause of foodborne illness. Antibiotic-resistant Salmonella can limit treatment options and constitutes a serious threat to public health. Salmonella is known to be able to acquire antimicrobial resistance genes (ARGs) through the transfer of mobile DNA called plasmids, and specific transfer genes in the DNA are necessary for the transfer event to occur. However, there is limited understanding what factors affect gene transfer between other bacteria and Salmonella. Salmonella can pick up ARGs from other bacteria present on the eggshell or bacteria in the environment. In this study, we evaluated the acquisition of ARGs by Salmonella from bacteria present on the eggshell or from the environment. This was tested under different conditions by orally inoculating 2-day old chicks with Salmonella sensitive to the antibiotic tetracycline and checking for the presence of Salmonella-resistant to the antibiotic tetracycline, which would indicate that the Salmonella acquired ARG from other bacteria. Salmonella colonized the chicks after inoculation, but tetracycline-resistant Salmonella were not recovered. Genome sequencing of bacteria from chickens before Salmonella challenge, which would be the potential bacterial donors of tetracycline resistance, showed they lacked the transfer genes needed for easy transfer to Salmonella plasmid DNA. The lack of plasmid transfer genes may explain why ARG exchange was not observed in this study. Sub-optimal conditions or genetic limitations may reduce the frequency of AMR transfer in animals. Our study offers new insights into the possible factors that can inhibit or lower the transfer efficiency of plasmids carrying ARGs to Salmonella in layer chickens.

Technical Abstract: Chicken ceca are a rich source of bacteria, including zoonotic pathogens such as Salmonella enterica. The microbiota includes strains/species carrying antimicrobial resistance genes and horizontal transfer of resistance determinants between species may increase the risk to public health and farming systems. The possible sources of these antimicrobial resistance donors – the eggshell carrying bacteria from the mother hen vertically transmitted to the offspring, or the barn environment where chicks are hatched and raised – has been little explored. In this study, we used Salmonella enterica serovar Heidelberg to evaluate if layer chicks raised in different environments (using combinations of sterilized or non-sterile eggs placed in sterilized isolation chambers or non-sterile rooms) acquired transferable tetracycline resistance genes from surrounding bacteria, especially Escherichia coli. Two-day old chicks were challenged with an antibiotic-susceptible S. Heidelberg strain SH2813nalR and Salmonella recovered from the cecum of birds at different timepoints to test the in vivo acquisition of tetracycline resistance. Tetracycline-resistant E. coli isolates recovered from birds from the in vivo experiment were used to test the in vitro transfer of tetracycline resistance genes from E. coli to Salmonella. Even though Salmonella SH2813nalR colonized the 2-day old chicks after oral challenge, tetracycline-resistant Salmonella transconjugants were not recovered, as previously observed. In vitro experiments provided similar results. We discuss several hypotheses that might explain the absence of transconjugants in vitro and in vivo, despite the presence of diverse plasmids in the recovered E. coli. The factors that can inhibit/promote antimicrobial resistance transfers to Salmonella for different plasmid types need further exploration.