Fire blight, caused by the gram-negative bacterium Erwinia amylovora, is a destructive disease of apple and pear trees worldwide. A unique aspect of apple and pear physiology is the presence of sorbitol in the leaves and shoots, glucose on the flower stigma surface and sucrose in the flower nectary. Erwinia amylovora cells encounter all three carbohydrates at different stages of infection, and it is unknown how the carbohydrate utilization genes are regulated between these changing nutrient environments. This thesis explores carbohydrate utilization by E. amylovora in relation to virulence, regulatory small RNAs (sRNAs), other virulence factors and host specificity. The findings presented here indicate that sorbitol utilization (srl) gene mutants of E. amylovora are amylovoran-deficient, and they are unable to obtain the energy base needed to infect apple shoots and immature pear fruits. Additionally, the sRNA Spot 42 does not regulate sorbitol as it does in Escherichia coli, and we hypothesize that E. amylovora has evolved to evade Spot 42 regulation in order to adapt to the high-sorbitol content of apple and pear hosts.
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