Date of Completion

11-16-2020

Embargo Period

11-16-2022

Keywords

Horizontal gene transfer, phytochemicals, Salmonella, E.coli, poultry

Major Advisor

Kumar Venkitanarayanan

Associate Advisor

Mary Anne Roshni Amalaradjou

Associate Advisor

Ann M Donoghue

Associate Advisor

Cameron L Faustman

Field of Study

Animal Science

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Bacterial antibiotic resistance (AR) imposes a huge threat to public health worldwide. Overuse and misuse of antibiotics in human medicine and animal agriculture resulted in the development of antibiotic resistant bacteria, including pathogens. Bacteria possess an inherent ability to share their genetic information by means of horizontal gene transfer (HGT), a property that favors dissemination of AR genes among neighboring bacteria. The animal gut, the reservoir of AR bacteria and AR genes, acts as a source for environmental contamination and propagation of AR through manure and water. There has been an increased incidence of AR Salmonella infections in humans in recent years. Most of the human infections are linked to direct and indirect contact with animals, especially poultry and farm environment. Therefore, an ideal approach for controlling AR Salmonella infections in humans would take animal and environmental components. Phytochemicals have gained interest as an alternative for antibiotics due to their antimicrobial properties, especially against AR bacteria. This Ph.D. dissertation investigated the potential of phytochemicals for their anti-HGT activity in the animal gut and farm environment for potential future application as an animal feed ingredient and as a manure and water treatment in animal production systems, respectively. The efficacy of phytochemicals, trans-cinnamaldehyde (TC), carvacrol (CR), caprylic acid (CA) and beta resorcylic acid (BR) in reducing the transfer of beta-lactamase gene (blaTEM) from multi-drug resistant Salmonella to a nonpathogenic E. coli in Luria-Bertani broth, animal manure and water was investigated. Additionally, the efficacy of TC and CR in reducing the HGT transfer frequency of beta-lactamase gene (blaTEM) in the invertebrate model, Caenorhabditis elegans, and chicken was determined. Furthermore, the phytochemical-mediated mechanisms of anti-HGT activity were investigated by conducting gene expression analysis of conjugation associated genes, followed by electron microscopic examination of Salmonella pili and by plasmid curing assay.

The results from in vitro studies in broth, manure, and water revealed a significant reduction in the transfer frequency of the blaTEM from Salmonella to E. coli in the presence of sub-inhibitory concentrations of the four phytochemicals. Moreover, the phytochemicals altered the expression of conjugation-associated genes in Salmonella, which was confirmed by a reduced piliation on Salmonella. In-feed supplementation of TC and CR also yielded reductions in HGT frequency in the C. elegans and chicken gut models. Collectively, these results suggest the potential use of aforementioned phytochemicals for controlling HGT-mediated AR dissemination in animal gut and environmental niches. However, follow-up studies with a large number of birds are warranted before recommending the application of phytochemicals for controlling AR spread in commercial poultry production.

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