Exopolysaccharide Production and Cell Aggregation in Azospirillum brasilense

Abstract

Azospirillum brasilense is a free-living, nitrogen-fixing, plant growth-promoting rhizobacterium that belongs to the alpha-proteobacteria. It closely associates with a large number of plants of agronomic importance, including grain and forage legumes, and cereal and forage grasses (Okon, 1994). A. brasilense strains are highly pleiomorphic and versatile in their metabolic activities in response to environmental changes. under unfavourable conditions, such as high oxygen partial pressure (Nur et al., 1981), desiccation and nutrient limitation, azospirilla can convert into ovoid, non-motile, encapsulated cyst-like forms (Sadasivan and Neyra, 1985). Similar changes occur when cells are grown in liquid minimal media supplemented with certain carbon sources, such as fructose or β-hydroxybutyrate (Bleakley et al, 1988). under these conditions, cells aggregate and flocculate in a matrix of polysaccharide material, forming large macroscopic clumps. Cell aggregation is a phenomenon of great interest for the production, storage, survival and adsorption to roots of bacterial inoculants for agricultural application.