Biofilm formation in Erwinia amylovora: Implications in pathogenicity

A biofilm is a surface-associated complex aggregation of bacteria, exopolysaccharides and macromolecules that may confer protection against harsh environments and also contribute to pathogenicity. Erwinia amylovora is a highly virulent, necrogenic, vascular pathogen of rosaceous species that produces the exopolysaccharide amylovoran, a known pathogenicity factor. A well established in vitro crystal violet staining method was used to show that E. amylovora is capable of forming a biofilm on solid surfaces. An amylovoran-biosynthesis deficient mutant with the 12-gene ams operon deleted was constructed and used to demonstrate that amylovoran production is necessary for biofilm formation. As previously shown, amylovoran is an essential pathogenicity factor for E. amylovora, and the δams mutant was asymptomatic and incapable of growth following inoculation in immature pear fruit. However, using a 50:50 mixture of the wild type E. amylovora and the δams mutant, growth of the δams mutant was rescued in co-infection experiments. The use of an in vitro flow cell, gfp- and cfp-labeled E. amylovora and confocal microscopy revealed further architectural detail of a mature biofilm. Scanning electron microscopy analysis of E. amylovora cells in apple xylem indicated that biofilm formation occurs in planta. These results demonstrate that the formation of a biofilm plays a critical role in the pathogenesis of E. amylovora.