Adhesion of Azospirillum brasilense: Role of proteins at the cell-support interface

The adhesion of the soil bacterium Azospirillum brasilense to polystyrene has been investigated using a parallel-plate chamber in conditions allowing cell transport to the support by sedimentation and rinsing under controlled hydrodynamic conditions. The adhesion pattern, which was heterogeneous, and the density of adhering cells were determined by cell aggregation at the support surface and detachment of aggregates upon rinsing. Determination of the support surface composition by X-ray photoelectron spectroscopy after detachment of adhering cells and analysis of the supernatant of cell suspensions revealed that, during the course of adhesion, extracellular proteins are released progressively into the aqueous phase and adsorb at the support surface. Support preconditioning by contact with a cell suspension promoted adhesion after a short contact time (2 h) due to protein adsorption at the support surface. Cell ageing prior to the adhesion test also enhanced adhesion after 2 h due to protein accumulation at the cell surface. Moreover, when supports were preconditioned and when cells were aged prior to the test, adhesion was still dependent on cell-support contact time, which pointed to the influence of in situ secretion of proteins by the adhering cells. It was therefore concluded that the role played by proteins at the cell-support interface is twofold. In the first stage, proteins accumulate at the cell surface, are liberated into the solution and adsorb at the support surface; the increase of the protein concentration at the interface promotes initial adhesion. In the next stage, in situ secretion of proteins during the prolonged contact between the cells and the support strengthens adhesion.