Location: Food Safety and Enteric Pathogens Research
2019 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 common source of bacterial foodborne-related illness, and between 1998-2016, pork products were the third highest associated food source. With greater than 50 percent of U.S. swine herds testing positive for Salmonella, carrier pigs that shed Salmonella in their feces without showing clinical symptoms are a challenging food safety issue and environmental contamination problem. The swine industry would benefit from a Salmonella-detection assay that reduces animal stress during sample collection and increases laboratory throughput. 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, a study was performed that evaluated the oral fluids (saliva) from pigs for antibodies against Salmonella. Oral fluids were collected from cotton ropes chewed on by groups of pigs before and after Salmonella exposure. Using an enzyme-linked immunosorbent assay (ELISA) developed in the laboratory, the oral fluid samples tested positive for Salmonella-specific antibodies. Additional research analysis is ongoing in support of oral fluids serving as a non-invasive sample source to detect anti-Salmonella antibody, thereby functioning as a valuable surveillance tool for herd level monitoring of Salmonella exposure.
Recent human foodborne outbreaks from contaminated pork products were caused by multidrug resistant (MDR) Salmonella enterica serovar I 4,[5],12:i:- isolates. To identify a cause for the increased prevalence of this particular Salmonella serovar, deoxyribonucleic acid (DNA) genome sequencing of a MDR Salmonella I 4,[5],12:i:- outbreak isolate was performed, and analysis revealed the acquisition of several genes involved in metal resistance (copper, silver, arsenic, and mercury). Because metals are frequently used in swine production as an antimicrobial feed additive, a study was conducted in pigs to evaluate the impact of standard, in-feed zinc and copper concentrations on MDR Salmonella I 4,[5],12:i:- colonization levels. In support of Objective 1 to investigate the impact of antibiotic usage on influencing Salmonella virulence mechanisms and enhancing antibiotic resistance, administration of high levels of zinc and copper in the diet did not reduce fecal shedding or tissue colonization of the Salmonella serovar I 4,[5],12:i:- isolate in pigs. In fact, at three weeks post-inoculation with the outbreak isolate, higher fecal shedding levels of Salmonella serovar I 4,[5],12:i:- were observed in the zinc/copper fed pigs compared to the pigs that did not receive high levels of zinc and copper in their feed. Thus, metals as an antimicrobial feed additive in the swine diet may have contributed to the persistence of Salmonella serovar I 4,[5],12:i:- in the pigs (i.e. selective pressure); current research studies are investigating the genetic elements acquired by the outbreak isolate that have contributed to its heightened metal resistance.
Accomplishments
1. Unintended consequences: therapeutic antibiotic treatment increased the amount of multidrug-resistant (MDR) Salmonella in pigs. Chlortetracycline is a commonly used antibiotic in veterinary medicine for treatment of respiratory and gastrointestinal infections. Many MDR Salmonella isolates are resistant to chlortetracycline, and chlortetracycline treatment for an infection may have unintended consequences in an animal unknowingly colonized with MDR Salmonella. ARS scientists in Ames, Iowa, revealed that pigs exposed to MDR Salmonella and given a therapeutic dose of chlortetracycline had higher levels of Salmonella in the oral cavity (tonsils) and feces when compared to pigs that did not receive chlortetracycline. Typically, Salmonella infections go unnoticed in food animals; Salmonella makes humans ill but does not make the food animal ill. Therefore, animals that are unsuspectingly colonized with MDR Salmonella and receive therapeutic chlortetracycline for an unrelated infection may shed more Salmonella to other pigs. In addition, the increased levels in pigs may increase environmental or pork product contamination, posing a risk to public health. Data indicates that consideration for Salmonella status may be important when administering therapeutic antibiotics.
Review Publications
Shippy, D.C., Bearson, B.L., Cai, G., Brunelle, B.W., Kich, J., Bearson, S.M. 2018. Modulation of porcine microRNAs associated with apoptosis and NF-kappaB signaling pathways in response to Salmonella enterica serovar Typhimurium. Gene. 676:290-297. https://doi.org/10.1016/j.gene.2018.08.044.
Bearson, S.M., Bearson, B.L., Sylte, M.J., Looft, T.P., Kogut, M.H., Cai, G. 2019. Cross-protective Salmonella vaccine reduces cecal and splenic colonization of multidrug-resistant Salmonella enterica serovar Heidelberg. Vaccine. 37(10):1255-1259. https://doi.org/10.1016/j.vaccine.2018.12.058.
Holman, D., Bearson, B.L., Allen, H.K., Shippy, D.C., Loving, C.L., Kerr, B.J., Bearson, S.M., Brunelle, B.W. 2018. Chlortetracycline enhances tonsil colonization and fecal shedding of multidrug-resistant Salmonella enterica serovar Typhimurium DT104 without major alterations to the porcine tonsillar and intestinal microbiota. Applied and Environmental Microbiology. 85(4):e02354-18. https://doi.org/10.1128/AEM.02354-18.
