Erwinia amylovora is a highly virulent, necrogenic plant pathogen that causes fire blight disease on apple, pear, and other rosaceous plants. The fire blight pathogen is highly invasive and capable of rapid systemic movement through plants. Current methods of control focus on chemical and antibiotic treatments. The popularity of highly susceptible cultivars and the emergence of antibiotic resistance have driven the need to find other methods to control fire blight. The study of pathogen virulence factors has the potential to identify novel control methods. Several bacterial virulence factors have been shown to be critical for biofilm formation, the production in a complex aggregated network of bacterial cells, exopolysaccharides, and other macromolecules. In addition, vascular plant pathogens commonly use the ability of biofilm formation to aid in the systemic movement of the pathogen. Research presented here used in silico analysis to identify several virulence factors. Biofilm formation and virulence assays determined that virulence factors that contribute to the systemic movement of E. amylovora in vascular tissue of apple. The production of exopolysaccharides amylovoran and levan was determined to be needed for the formation of a mature biofilm. Though cells are motile, amylovoran deficient mutants are unable to move past the site of inoculation. Levan deficient mutants display a delayed, reduced virulence phenotype. Several putative bacterial surface proteins, or attachment structures, assist in the initial attachment (both reversible and irreversible) of the pathogen to host tissue. Deletions in genes encoding for the production of attachment structures drastically reduce the biofilm capability of E. amylovora, and the ability to get into the xylem tissue. In addition, mutation of functional flagellar motor stators demonstrated that motility was important in mediating contact of bacterial cells to surfaces. Motility of the fire blight pathogen is shown to be important in the movement of bacterial cells within host tissue. Additionally, motility seems to be important in the expansion of biofilms. In total, the contribution of the virulence factors to biofilm formation and the localization of the pathogen in host tissue imply that biofilm formation assists in the systemic movement of E. amylovora in apple.
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