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Effect of Increased β-Glucosidase Activity on Virulence of Erwinia amylovora

Plant tissues often contain β-glucosides that can be enzymatically hydrolyzed to produce toxic aglycones. It has been suggested that the low β-glucosidase activity found in Erwinia amylovora contributes to bacterial virulence by allowing the bacteria to infect plants that contain β-glucosides without inducing the formation of toxic aglycones. To test this suggestion, we created strains of E. amylovora which had high β-glucosidase activities and studied the ability of these strains to cause fire blight disease in pears (Pyrus communis). We isolated spontaneous mutants that were able to utilize β-glucosides as the sole carbon source and showed that one class had about 10 times as much β-glucosidase activity as the wild-type strain. In addition, we constructed several plasmids that carry the Escherichia coli bgl operon under the control of a transposon Tn5 promoter that is expressed in E. amylovora. These plasmids were introduced in E. amylovora by transformation. Pathogenesis studies in immature Bartlett pear fruits, etiolated sprouts, and young shoots showed that a 100-fold increase in β-glucosidase activity does not interfere with normal development of fire blight disease in these model systems.