Dissecting the process of xylem colonization through biofilm formation in Erwinia amylovora

Erwinia amylovora is the causal agent of fire blight, an economically-important disease affecting apple and pear production worldwide. Initial contact and infection of the host by E. amylovora mainly occurs in flowers, or in young leaves at actively-growing shoot tips. Infection via shoot tips encompasses several distinct steps which include the utilization of a Type III secretion system (T3SS) to establish bacterial populations within the apoplast, infection of the parenchyma, invasion of the xylem, attachment to xylem vessels, biofilm formation, and the eventual colonization of the xylem which manifests outwardly as wilting symptoms in the plant. After E. amylovora gains entry into the xylem, initial attachment to the xylem vessels is mediated by type I fimbriae. Conversely, the small RNA (sRNA) chaperone Hfq and associated sRNA ArcZ negatively regulate attachment and promote biofilm maturation. Attachment and biofilm formation within the xylem are enhanced by the mechanical force emerging from the flow of xylem sap. The second messenger molecule cyclic-di-GMP (c-di-GMP) regulates the transition into the biofilm phase of the infection process of E. amylovora. C-di-GMP also regulates the production of critical exopolysaccharides amylovoran and cellulose, that lend to the structural stability and growth of biofilms within the xylem vessels. In this review, we provide an in-depth evaluation of the process of biofilm formation occurring within the host, as a result of E. amylovora infection. We also provide a model encompassing the different physical and signaling factors involved in biofilm initiation and maturation in E. amylovora, and highlight what needs to be done in the future.