Exploring the Temporal Dynamics of Salmonella enterica and Bacteriophage Applications in the Feedlot Environment for Mitigating Salmonella enterica in Beef Cattle

Abstract

Human salmonellosis is a common foodborne illness that can result from contaminated food, including beef products. Bovine lymph nodes, which may harbor Salmonella, are difficult to separate from adipose tissues that are commonly incorporated into ground beef. The current lack of post-harvest interventions addressing this food safety concern calls for alternative pre-harvest Salmonella mitigation techniques. A three-part research study, including; longitudinal sampling, proof-of-concept, and an experimental trial, were conducted to determine the effectiveness of pre-harvest bacteriophage treatments at mitigating or shifting Salmonella populations to favor pan-susceptible varieties in the feedlot environment and in beef cattle. After Salmonella colonization of the bovine digestive tract, fecal shedding can perpetuate contamination of the feedlot pen environment. To study these cyclical dynamics, and their relationship to Salmonella in bovine lymph nodes, pen environment (pen surface composite, water, feed) and bovine samples (feces and lymph nodes) were collected for a 7-month longitudinal comparison of Salmonella prevalence, serovars, and antimicrobial resistance (AMR) profiles. Results suggest that Salmonella serovar composition differs between the environment and host. However, phylogenetic analyses highlight examples of Salmonella isolates originating from cattle and their associated feedlot pen that are frequently closely related. Our proof-of-concept study identified naturally occurring, Salmonella-infecting bacteriophages in the feedlot pen environment, suggesting they could be harnessed and applied to cattle and the environment as a treatment. Additionally, bacteriophage cocktails can be selected to target specific Salmonella serovars of food safety concern. During an experimental trial, we tested a naturally occurring bacteriophage treatment applied with dust abatement pen sprinklers and a lab-produced bacteriophage cocktail applied with battery operated backpack sprayers in a 2 x 2 factorial design for their effectiveness at mitigating Salmonella in the feedlot (pen environment) and in cattle (feces, brisket hide, rump hide, and lymph nodes). We found that both natural and targeted phage treatments were effective at reducing Salmonella on cattle hides, suggesting there is potential to also reduce Salmonella in cattle lymph nodes; however, further research and development needs to be performed. This research would aid in determining if bacteriophage applications may be an effective pre-harvest intervention for reducing Salmonella prior to their dissemination into cattle, addressing the lack of post-harvest Salmonella mitigation approaches suited to lymph nodes.