Seminar: Nutrient Acquisition and Pathogenicity in Erwinia amylovora

Melissa Finley, M.S. Student, Penn State University
Melissa Finley | Image: Michael Houtz, Penn State

Melissa Finley | Image: Michael Houtz, Penn State

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April 25, 2017, 1:00 PM - 2:00 PM

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The gram-negative bacterium Erwinia amylovora is the causal agent of fire blight, a destructive disease of apples, pears, and other Rosaceae species. This study seeks to further elucidate the trophic aspects of the host-pathogen parasitic interaction and how specific metabolic biosynthetic pathways may be required for pathogenicity. Auxotrophic mutants were generated via Tn5 transposon mutagenesis and identified with a selective minimal media on which auxotrophs could not grow. Auxotrophic mutants were then inoculated onto immature ‘Gala’ apple fruits in order to monitor differences in pathogenicity caused by the Tn5 mutations. The mutated genes were then identified by Sanger sequencing of E. amylovora DNA flanking the Tn5 insertion in each auxotrophic mutant. The transposon insertions were found to have affected the following major classes of biosynthetic pathways: amino acid biosynthesis, nucleotide biosynthesis, sulfur metabolism, nitrogen metabolism, survival protein biosynthesis, exopolysaccharide biosynthesis, and a selection of uncharacterized proteins. If an auxotrophic mutant is able to cause disease despite the metabolic defect, this indicates that it must be able to derive the missing metabolites from host tissues. If an auxotrophic mutant is not able to cause disease, this suggests that it cannot derive the missing metabolites from their host. It was determined that the disruption of amino acid biosynthesis, such as for the production of arginine, leucine, and methionine, and nucleotide biosynthesis, such as for the production of purines and pyrimidines, results in reduced or asymptomatic disease expression. This indicates that mutants with these disrupted pathways are unable to obtain sufficient amounts of the missing metabolic products from the host tissue in order to complement their metabolism and grow normally. Conversely, mutants with disrupted sulfur metabolism remained pathogenic, indicating that these mutants were able to obtain sufficient amounts of sulfur and sulfur metabolites from the host tissue in order to complete the biosynthetic pathway and grow normally. In addition, several survival protein biosynthesis and exopolysaccharide biosynthesis mutants were identified during screening as possible auxotrophs, and these displayed reduced or absent disease expression. The reason for these mutants being identified as auxotrophs is still being investigated. This study reveals new details of the profile of pathogen-accessible metabolites in the host tissues and how that may aid or hinder bacterial growth processes as disease develops.

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