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ARS Home » Midwest Area » Ames, Iowa » National Animal Disease Center » Food Safety and Enteric Pathogens Research » Research » Research Project #430777

Research Project: Analysis of Virulence and Antibiotic Resistance Mechanisms of Salmonella and Development of Intervention Strategies

Location: Food Safety and Enteric Pathogens Research

2018 Annual Report

1a. Objectives (from AD-416):
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.

1b. Approach (from AD-416):
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.

3. Progress Report:
Salmonella is a major foodborne pathogen, and several of the recent foodborne outbreaks associated with Salmonella contamination of pork products have been caused by multidrug resistant (MDR) monophasic Salmonella I 4,[5],12:i:- isolates. In order to evaluate the pathogenicity of MDR Salmonella I 4,[5],12:i:- and in support of Objective 1 to investigate the impact of antibiotic usage on influencing Salmonella virulence mechanisms and enhancing antibiotic resistance, a study was conducted whereby pigs were inoculated with a MDR Salmonella I 4,[5],12:i:- isolate from a 2015 outbreak associated with pork products. The Salmonella I 4,[5],12:i:- inoculated pigs exhibited transient clinical disease and Salmonella fecal shedding that peaked 2-3 days after inoculation, which is a similar presentation observed in swine challenged with other Salmonella strains. Thus, increased virulence in the natural porcine host is not a likely reason for the increase in MDR monophasic Salmonella I 4,[5],12:i:- prevalence in swine or contamination of pork products. Research is ongoing to evaluate environmental persistence mechanisms of Salmonella I 4,[5],12:i:- as a potential cause for its increased prevalence in pigs. Salmonella enterica serovar Heidelberg has been isolated from most food-producing animals, shown increased resistance to antimicrobial agents, and is among the top five serovars associated with human foodborne illness. In support of Objective 2 to develop novel non-antibiotic intervention strategies to limit Salmonella colonization, persistence and shedding, turkey poults (one-day old) were inoculated with a MDR Salmonella Heidelberg isolate from the 2011 ground turkey outbreak, and transmission to non-inoculated penmates was monitored. The isolate was able to colonize and transmit to non-inoculated poults, with colonization levels reaching 100-fold above the inoculum dose over a one week period. The colonization pattern was different in older turkeys (three-weeks old) that were inoculated with an even higher dose of Heidelberg; in the older turkeys, tissue colonization and fecal shedding of the Heidelberg isolate decreased over a two week period. The efficient transmission, colonization, and proliferation of MDR Heidelberg during the first week of life suggests that limiting the introduction of Salmonella into turkey flocks during the establishment of the intestinal microbiota is critical for control of this human foodborne pathogen. Current research is focusing on vaccine efficacy, evaluating different experimental vaccine platforms as an intervention strategy to prohibit Salmonella from establishing a commensal-like condition in poultry, and thus reduce the asymptomatic carrier status of the human foodborne pathogen in our animal food supply.

4. Accomplishments
1. Antibiotic exposure can induce the expression of disease-associated genes in multidrug-resistant (MDR) Salmonella. Chlortetracycline and florfenicol are antibiotics commonly used in veterinary medicine for respiratory and gastrointestinal infections, but the dose used to eliminate infections are not inhibitory to many MDR Salmonella isolates (i.e. sub-inhibitory). ARS scientists in Ames, Iowa revealed that exposure of MDR Salmonella isolates to either chlortetracycline or florfenicol changed the expression levels of over 50 percent of Salmonella genes, including genes involved in the ability of Salmonella to move (motility), invade host cells (invasion), and survive and replicate inside host cells (virulence). Salmonella typically does not cause clinical symptoms nor disease in food animals (poultry, swine, cattle); thus, food animals can be unknowingly colonized with MDR Salmonella. Treatment of an unrelated disease can expose the colonizing MDR Salmonella to sub-inhibitory concentrations of the antibiotic, thereby potentially enhancing expression of Salmonella virulence genes and prolonging host colonization and fecal shedding of this human foodborne pathogen. Understanding the potential collateral effects of antibiotics on Salmonella will provide information to practitioners to help limit the negative consequences of antibiotic therapy.

Review Publications
Bearson, B.L., Bearson, S.M., Looft, T., Cai, G., Shippy, D.C. 2017. Characterization of a multidrug-resistant Salmonella enterica serovar Heidelberg outbreak strain in commercial turkeys: Colonization, transmission, and host transcriptional response. Frontiers in Veterinary Science. 4:156.

Brunelle, B.W., Bearson, B.L., Bearson, S.M., Casey, T. 2017. Multidrug-resistant Salmonella enterica serovar Typhimurium isolates are resistant to antibiotics that influence their swimming and swarming motility. mSphere. 2:e00306-17. https://doi.10.1128/mSphere.00306-17.

Holman, D., Bearson, S.M., Bearson, B.L., Brunelle, B.W. 2018. Chlortetracycline and florfenicol induce expression of genes associated with pathogenicity in multidrug-resistant Salmonella enterica serovar Typhimurium. Gut Pathogens. 10:10.

Shippy, D.C., Bearson, B.L., Holman, D.B., Brunelle, B.W., Allen, H.K., Bearson, S.M. 2018. Porcine response to a multidrug-resistant Salmonella enterica serovar I 4,[5],12:i:- outbreak isolate. Foodborne Pathogens and Disease. 15(5):253-261.