DIRECT EVIDENCE FOR THE INVOLVEMENT OF EXTRACELLULAR PROTEINS IN THE ADHESION OF AZOSPIRILLUM-BRASILENSE

Adhesion of Azospirillum brasilense to glass and polystyrene was inves tigated by bringing the cells into contact with the support by sedimen tation. Adhesion depended on time and temperature: lower adhesion dens ities were observed when the contact time was only 2 h or 6 h, as comp ared to 24 h, or when the test was performed at 4 degrees C, as compar ed to 30 degrees C. The influence of cell physiology was further demon strated by the effect of tetracycline, which inhibited adhesion. Scann ing electron microscopy showed that cells produced extracellular mater ial when left in contact with a support for 24 h. The surface elementa l composition of cells and of polystyrene supports after cell adhesion and subsequent detachment was determined by X-ray photoelectron spect roscopy; this provided information on the relative concentrations of p roteins and polysaccharides at the surface. The protein concentration at the surface of a cell sediment increased as a function of time at 3 0 degrees C, correlating with an increase of adhesion density. A simil ar correlation between protein concentration and adhesion density was found when comparing exponential-phase cells with stationary-phase cel ls. The surface composition of polystyrene supports examined after cel l detachment was found to be rich in proteins, indicating that protein s are the major constituent at the support surface. Lowering the conta ct time, or performing adhesion under unfavourable metabolic condition s (4 degrees C) or in the presence of tetracycline, resulted in a decr ease in protein concentration at the support surface, which was correl ated with a decrease in adhesion density. The correlation between prot ein concentration at the cell surface or at the support surface and ad hesion density, under different experimental conditions, provides a di rect demonstration of the involvement of extracellular proteins in the adhesion of A. brasilense to inert surfaces.