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United States Department of Agriculture

Agricultural Research Service

2010 Annual Report

1a.Objectives (from AD-416)
(1) Investigate the molecular mechanisms that coordinate virulence and antibiotic resistance in Salmonella obtained from cattle (DT104 and S. dublin) and swine (DT104 and S. choleraesuis). (2) Characterize the molecular mechanisms involved in Salmonella survival, colonization and pathogenicity in relation to various host (swine and cattle) factors such as stress, gastric pH, and protozoa. (3) Investigate the molecular basis for swine resistance to Salmonella colonization by characterizing the immunological aspects of infection.

1b.Approach (from AD-416)
(1) Use a novel reporter system and transposon mutagenesis o identifiy DT104 genes involved in rumen protozoa-mediated hypervirulence. (2) Use an in vitro conjugation system to chracterize intra-protozoal transfer of plasmids from Klebsiella (and other bacteria) to Salmonella. (3) Compare the progression of salmonellosis for defaunated and fully-faunated calves infected with DT104. (4) Functional genomic analyses will be performed on Salmonella exposed to norepinephrine, low pH, and conditions relevant to intestinal colonization. (5) Use microarrays to identify swine genes that contribute to innate resistance to Salmonella colonization.

3.Progress Report
We studied the ability of Salmonella to sense and rapidly respond to changes in their environment using the quorum sensing, two-component signal transduction system QseBC. In this two-component signaling system, one protein (QseC) serves as the signal sensor while another protein (QseB) coordinates the bacterial response to the signal. Our investigations determined that the regulator protein (QseB) decreases bacterial motility and virulence; this negative regulation is controlled by the sensor protein (QseC). When the sensor protein is removed, Salmonella has decreased colonization in the gastrointestinal tract of pigs. This information provides a genetic target (qseBC genes) and a functional mechanism (quorum sensing) for developing control strategies against Salmonella. Isolates of Salmonella Typhimurium DT104 are resistant to five or more antibiotics that typically include ampicillin, chloramphenicol, florfenicol, streptomycin, sulfonamides, and tetracycline. To determine if antibiotics can enhance virulence of S. Typhimurium DT104, isolates were grown in the presence or absence of florfenicol under low oxygen conditions, and gene expression levels of a critical invasion factor were determined. Results indicate that florfenicol can increase both the severity and duration of virulence in S. Typhimurium DT104. Future work will assess the effect(s) of the other antibiotics on S. Typhimurium DT104 virulence, thereby establishing the role of antibiotics in enhancing Salmonella pathogenesis in cattle and swine.

1. Association of Salmonella Shedding in Pigs with Genetic Variation in Salmonella-Response Genes. An alternative to antibiotic usage in food-producing animals is to select for animals with improved genetic resistance to zoonotic pathogens. An ARS researcher at Ames, IA in cooperation with Iowa State University scientists has searched sequence databases for genetic variations in greater than 3,000 porcine genes identified by the research team as differentially-expressed during exposure to Salmonella. Thirty DNA sequence variations in the pig genome, referred to as single nucleotide polymorphisms (SNPs), were identified. Genotype analysis of four independent pig populations revealed that three of the SNPs associated with fecal shedding or tissue colonization of Salmonella in pigs. Classifying porcine genetic differences that associate with Salmonella shedding status will advance the identification and breeding of pigs with decreased colonization and fecal shedding of Salmonella.

Review Publications
Bearson, B.L., Bearson, S.M., Lee, I., Brunelle, B.W. 2010. The Salmonella enterica serovar Typhimurium QseB Response Regulator Negatively Regulates Bacterial Motility and Swine Colonization in the Absence of the QseC Sensor Kinase. Microbial Pathogenesis. 48:214-219.

Last Modified: 11/27/2015
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