Context matters for antibiotic resistance horizontal gene transfer from commensal bacteria to Salmonella in the chicken gastrointestinal tract

Authors: Looft, Torey; CASSIDY, KLIMA - Beef Cattle Research Council; GUERNIER-CAMBERT, VANINA - Pierre And Marie Curie University; Trachsel, Julian

Submitted to: Keystone Symposia
Publication Type: Abstract Only
Publication Acceptance Date: 12/4/2022
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Antimicrobial resistance (AMR) in human pathogens has made treatment of infections a significant problem. AMR Salmonella is a serious concern, as is the potential for AMR genes to transfer between commensals and pathogens. The objectives of this study were to determine which AMR genes could be acquired by Salmonella enterica serovar Heidelberg SH2813 from the gut microbiota in vivo, and to identify potential microbial AMR donors. Forty day-old chicks were orally inoculated with 2X10^8 cfu of nalidixic acid resistant Salmonella. Ten birds were euthanized at 1, 2, 4, and 6 weeks of age, and cecal contents were collected for total Salmonella enumeration as well as determination of acquired tetracycline or ampicillin resistance. Salmonella initially colonized chicks at high levels, but declined with each subsequent sampling timepoint. A high percent of the Salmonella recovered were tetracycline resistant. Sequence data from plasmids acquired by SH2813 showed diversity of plasmid types and AMR genes. Plasmid annotations suggested some of the acquired plasmids may have originated from E. coli. A second animal trial was conducted to determine if the AMR donors were present on the eggshell prior to hatch. Eggs were hatched conventionally (germ-replete) or after disinfection, in non-sterile rooms; and a set of conventional eggs was hatched in a sterile isolator (n=36 per group). Three days after hatch, chicks were orally inoculated with Salmonella (SH2813) as described above, euthanized at 1, 2, and 3 weeks, and cecal contents were collected for Salmonella enumeration as well as determination of acquired tetracycline resistance. However, no AMR transfers were detected in SH2813 isolated from chicks despite Salmonella colonization and the presence of tet resistant E. coli in examined cecal contents. In vitro conjugation experiments confirmed the lack of tet transfer from E. coli isolates to SH2813. Genome assemblies from the E. coli strains confirmed the tet genes were chromosomally located, or were found on plasmids that lacked transfer genes. Ongoing research aims to explore the genetic diversity of antibiotic resistance reservoirs within the microbiota and the ability for genes to be acquired by Salmonella from other microbes. Understanding factors that impact commensal bacteria transferring resistance genes to pathogens will inform interventions to slow this process.