Location: Food Safety and Enteric Pathogens Research
2020 Annual Report
Objectives
Our objectives target three factors that influence Salmonella colonization, pathogenesis, and persistence. These factors include virulence mechanisms of Salmonella, the tactical response from the host, and interactions with the microbiota residing within the host. Our systematic approach integrates these research areas into three complementary objectives:
Objective 1: Investigate the impact of antibiotic usage on influencing Salmonella virulence mechanisms and enhancing antibiotic resistance.
Objective 2: Develop novel non-antibiotic intervention strategies such as beneficial microbes and vaccines to limit Salmonella colonization, persistence and shedding.
Objective 3: Evaluate immune networks and identify porcine genes for their relationship with the host microbiota to reduce Salmonella colonization, persistence, and shedding.
Approach
The common goal of each research objective is to identify targets for the development of novel antibiotic alternatives to reduce both Salmonella transmission through the food chain and antibiotic usage on the farm. To accomplish these objectives, experiments are planned to examine molecular mechanisms in Salmonella that are influenced by antimicrobial resistance and host colonization, elucidate porcine genetic pathways associated with decreased Salmonella colonization, and investigate interactions between Salmonella and host microbiota that affect Salmonella colonization and persistence. We plan to: 1) identify antibiotics that enhance virulence properties in multidrug-resistant (MDR) Salmonella, as well as those antibiotics that have no effect on virulence; this useful information will aide producers and veterinarians when determining antibiotic therapy for the treatment of infectious diseases; 2) genome sequence and transcriptionally analyze MDR Salmonella isolates that phenotypically respond to antibiotic exposure; 3) measure the effect chlortetracycline treatment has on limiting or exacerbating Salmonella shedding and altering the microbiota in swine; 4) evaluate a cross-protective Salmonella vaccine in turkeys for reduction of Salmonella colonization and transmission; 5) systematically characterize changes in the porcine immune response and gastrointestinal microbiota during Salmonella colonization; 6) assess biotherapeutic treatments as alternatives to antibiotics for treatment of swine colonized with Salmonella.
Progress Report
Salmonella is a bacterium that can cause human food-borne illness after transmission to humans from food animal products, including pork. Approaches to improve food safety include development of methods to reduce Salmonella in food animals. In order to develop these methods it is necessary to understand how Salmonella invades and resides in the animal host. In support of Objective 3 to “evaluate immune networks and identify porcine genes for their relationship with the host microbiota to reduce Salmonella colonization, persistence and shedding,” samples of lymphatic tissue, called the Peyer’s Patch region, of the ileum (the preferred site of Salmonella invasion in the small intestines) were collected from pigs during the acute stage of infection (1, 2 and 3 days after challenge). Tissue samples were processed, and an immunohistochemistry assay to detect Salmonella in the tissues was optimized. Detection of Salmonella in tissues is an important step in identifying mechanisms involved in invasion and colonization of the intestinal tissue. These data, in combination with the transcriptional response of the pig and changes in the intestinal microbial populations (microbiota) during acute infection, will support our research program to identify and exploit unique porcine-microbiota-Salmonella interactions to identify novel intervention targets.
Non-antibiotic intervention strategies that control bacterial pathogens while benefitting animal health are a desire of food animal producers. Beta-glucan is a soluble dietary fiber that has been marketed as beneficial for gut health and immunity. In support of Objective 2 to “develop novel non-antibiotic intervention strategies such as beneficial microbes and vaccines to limit Salmonella colonization, persistence and shedding,” a study was conducted with an industry collaborator whereby pigs were fed a diet with and without yeast-derived beta-glucan and inoculated with Salmonella enterica serovar Typhimurium to determine if beta-glucan influences Salmonella colonization. A reduction in Salmonella colonization was observed in some of the intestinal tissues of the pigs fed beta-glucan compared to the pigs that did not receive beta-glucan. Investigation of the pigs’ microbiota (bacteria represented in the gut) using 16S rRNA sequencing identified significant microbial community differences between the beta-glucan treatment groups. Current research is evaluating the bacteria enriched in the intestinal microbiota of the beta-glucan fed pigs for potential beneficial effects such as reducing Salmonella in the swine gastrointestinal tract.
Accomplishments
1. Swine saliva can signal Salmonella exposure. Greater than 50 percent of U.S. swine herds are positive for the human food-borne pathogen Salmonella. Because pigs are often colonized with Salmonella without showing clinical signs (i.e., carriers), an important first step in reducing the incidence of Salmonella on the farm is to promptly and easily detect when a herd has been exposed. The swine industry would benefit from a surveillance tool that would identify Salmonella-exposed pigs and determine herd level immunity. ARS scientists in Ames, Iowa, revealed that oral fluids (i.e., saliva) from pigs could be used to detect antibodies against Salmonella. Oral fluid samples are collected by allowing pigs to chew on cotton ropes, which is non-invasive, inexpensive, and can be repeatedly collected from live animals. The samples can be easily stored and used in a laboratory assay commonly performed in diagnostic labs to detect the presence of Salmonella-specific antibodies. The data support oral fluids serving as a repeatable sample during swine production to provide not only timely surveillance information on Salmonella exposure and herd immunity, but also to evaluate the effectiveness of disease intervention strategies against Salmonella.
Review Publications
Atkinson, B.M., Bearson, B.L., Loving, C.L., Zimmerman, J.J., Kich, J.D., Bearson, S.M. 2019. Detection of Salmonella-specific antibody in swine oral fluids. BMC Porcine Health Management. 5(29). https://doi.org/10.1186/s40813-019-0136-7.
Bearson, B.L., Trachsel, J.M., Holman, D.B., Brunelle, B.W., Sivasankaran, S.K., Simmons, M., Wasilenko, J., Tillman, G., Johnston, J.J., Bearson, S.M. 2019. Complete genome sequence of multidrug-resistant Salmonella enterica serovar I 4,[5],12:i:- 2015 U.S. pork outbreak isolate USDA15WA-1. Microbiology Resource Announcements. 8(40):e00791-19. https://doi.org/10.1128/MRA.00791-19.
