Extracellular factors involved in biofilm matrix formation by Rhizobium leguminosarum

ABDIAN, P.L.

Essen

Conferencia; The Perfect Slime; 2014

Rhizobia are soil bacteria that play an important role in biological nitrogen fixation. Under conditions of nitrogen starvation, they establish symbiotic interactions with leguminous plants, inducing the formation of root nodules. Inside these nodules the bacteria, in the form of bacteroids, convert atmospheric nitrogen to ammonia. Rhizobia can also exist as free-living microorganisms in the rhizosphere and bulk soil. It has been recognized that biofilm formation is a key process in rhizobial complex lifestyle. The long known cap structure on root tips is indeed a biofilm, that also rely on plant components and is formed by different specialized mechanisms according to prevailing pH conditions. On the other hand we showed that the biofilm formed in vitro on glass surfaces requires different determinants than biofilm formation on root hairs, and it was proposed that the former may reflect the type of biofilm formed on soil particles. The survival of rhizobia in the bulk soil is of great importance, since increasing rhizobial populations would enhance the probability of plant infection. We studied Rhizobium leguminosarum biofilm formation in vitro and identified several extracellular factors. Among them, the acidic polysaccharides (exo- and capsular polysaccharides) as well as a family of extracellular proteins, termed Raps (Rhizobium adhering proteins) are key factors for biofilm formation. We analyzed the isolated macromolecules that compose the EPS, defining some of their roles and their relationship in the context of biofilm development. By means of genetic, biochemical and biophysical studies on isolated components, AFM imaging of polysaccharide and observations on biofilm formation by immunofluorescence and CLSM, we are gaining insight in the mechanistic aspects of matrix formation. We propose that a particular organization of the acidic exopolysaccharide forming ordered meshes provides the structural basis for R. leguminosarum biofilm development, that these meshes depend on the correct processing of polysaccharide, and that the Raps are directly involved in the modulation of the polysaccharide matrix.