Early succession of bacterial communities associated as biofilm-like structures in the rhizosphere of alfalfa

Abstract

Many processes of crucial ecological, interactive and productive importance take place between plants and microorganisms in the rhizosphere, the portion of soil closest to plant roots. Yet, little is known about the establishment, succession and functions of bacterial communities in the rhizosphere which are dependent on the development of biofilm structures. This work used physiological and molecular characterization to study changes in bacterial biofilm communities in the rhizosphere of alfalfa (Medicago sativa L.). We evaluated natural biofilm-like structures and the early succession of artificial biofilm-like structures associated to rhizospheric soil by building a simplified community with bacterial strains isolated from artificial surfaces under laboratory conditions. Comparing the artificial succession assay with natural rhizospheric soil, results showed altered bacterial counts, biofilm-forming abilities and community structures at different time points. Highly adhesive strains belonging to γ-proteobacteria dominated the initial stages of biofilm development associated to roots, whereas α-proteobacteria and Actinobacteria prevailed in naturally established mature biofilms in the rhizosphere. Adhesion assays on the roots showed changes in the structure and dynamics of the bacterial community after short experimentation times, which correlated quite well with the results obtained on artificial surfaces. Summing up, the study found that the establishment of multibacterial communities in the rhizosphere of alfalfa is a dynamic process which probably includes the initial formation of biofilm-like structures by highly-adherent strains and the subsequent shaping towards a mature community through mechanisms of replacement and co-existence.