Biofilm formation by Bradyrhizobium was first described by Sereviratne and Jayasingherachchi (2003). Since then, both bacterial and plant surface molecules have been shown to be involved in the establishment of microbial communities on legume roots. In the symbiosis between Bradyrhizobium PR-171 purchase sp. and peanut plant, the attachment level varies

depending on the metabolic state of the rhizobia. Optimal attachment was observed when cells were harvested at the late log or the early stationary phase of growth (Dardanelli et al., 2003). A 14-kDa calcium-binding protein is important for bacterial attachment to the plant root, because root incubation with this adhesin before the attachment assay resulted in a significant, dose-dependent decrease of attachment. EDTA treatment of the cells caused the release of the rhicadhesin-like protein from the bacterial surface into the culture medium, and bacterial attachment was restored (Dardanelli et al., 2003). Plant lectins are proteins that reversibly and nonenzymatically bind specific carbohydrates (De Hoff GDC-0449 et al., 2009). They play important roles during the early stages of interaction

between the host plant and the symbiotic bacteria, particularly in the initial attachment of rhizobia to root epidermal cells. Soybean lectin causes a dose-dependent increase of attachment and biofilm formation on polystyrene surface by Bradyrhizobium japonicum wild-type USDA 110 cultures (Pérez-Giménez et al., 2009). Preincubation of rhizobia with soybean lectin increases bradyrhizobial adhesion to soybean roots (Lodeiro et al., 2000). Exopolysaccharides seem to be involved in B. japonicum biofilm formation on both inert and biotic surfaces (Pérez-Giménez et al., 2009). A mutant, which lacks UDP-Glc-4′ epimerase activity and produces second low levels of a shorter exopolysaccharide lacking galactose, showed biofilm biomass less than that of the wild-type strain. The defective phenotype was not

restored by soybean lectin addition to the mutant culture. Adhesion of mutant cells to soybean roots was significantly lower than that of the wild-type strain, indicating that complete exopolysaccharide is required for efficient colonization of B. japonicum on soybean (Pérez-Giménez et al., 2009). Attachment of R. leguminosarum to plant root hairs has two steps: primary attachment mediated by either bacterial adhesins (Smit et al., 1992) or plant lectins (Dazzo et al., 1984) and then secondary attachment via cellulose fibrils on the bacterial surface (Dazzo et al., 1984). Rhizobium leguminosarum, like many other bacteria, forms biofilms on sterile inert surfaces (Fujishige et al., 2005, 2006). The biofilm formation ability, assessed by a microtiter plate assay, was much lower in a pSym-deficient mutant than in R. leguminosarum bv.