Adsorption of Azospirillum brasilense to corn roots

The adsorption of Azospirillum brasilense strain Cd to corn roots was measured using ³²P-labeled Azospirillum cells. Binding kinetics showed that adherence of bacteria to roots increased during the first 90 min and attained a maximum level within 4.5 h. Maximum binding occurred at pH 6.1. The presence of Ca²⁺, Mg²⁺, Na⁺ or poly-l-lysine (a polycation) reduced binding. Bacterial adherence to roots increased with increasing bacterial concentration up to 10⁹ cells ml^-1 of binding mixture. At higher concentrations, binding approached saturation. The bacteria attached to the roots and the bacteria in the solution around the roots were found to be in a dynamic equilibrium. Killing bacteria or roots with heat or glutaraldehyde inhibited the attachment. High concentrations of bacteria in the binding mixture (10⁹-10¹¹ cells ml^-1) resulted in binding of bacteria to the roots in aggregates. The presence of several sugars, amino acids and organic acids inhibited adsorption. Chemotaxis of Azospirillum towards corn root exudates was observed. Treating the extract with trypsin eliminated this effect. At pH 6.1, Azospirillum brasilense strain Cd adsorbed in higher numbers than Klebsiella pneumoniae, Pseudomonas flourescens and Escherichia coli. Differences in adsorption levels were also found among several strains and mutants of Azospirillum.