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The plant-beneficial bacterium Azospirillum brasilense can swim in liquids and swarm or migrate with the formation of microcolonies in soft media. To get closer to understanding the influence of natural environments on A. brasilense motility, we studied the individual and social movement of the bacterium in the presence of various plant lectins. The lectins with specificity for N-acetyl-beta-d-glucosamine oligomers (wheat germ, Solanum tuberosum and Ulex europeus agglutinins) decreased A. brasilense swimming speed and induced the formation of branched-granular colonies instead of the swarming rings. These effects seemed to be a consequence of specific interactions between the agglutinins and the lectin-binding polymers present in the A. brasilense cell envelope. Concanavalin A (with an affinity for terminal alpha-d-mannosyl and alpha-d-glucosyl residues) and Phaseolus vulgaris phytohemagglutinin P (with unknown specificity) almost did not affect the motility of A. brasilense.

The rhizobacterium Azospirillum brasilense Sp245 swims, swarms (Swa(+) phenotype) or, very rarely, migrates with the formation of granular macrocolonies (Gri(+) phenotype). Our aims were (i) to identify Sp245 mutants that swarm faster than the parent strain or differ from it in the mode of spreading and (ii) to compare the mutants' responses to wheat seedling exudates. In isotropic liquid media, the swimming speeds of all motile A. brasilense strains were not influenced by the exudates. However, the exudates significantly stimulated the swarming of Sp245. In several Sp245 mutants, the superswarming phenotype was insensitive to local colonial density and to the presence of wheat seedling exudates. An A. brasilense polar-flagellum-defective Gri(+) mutant BK759.G gave rise to stable Swa(++) derivatives with restored flagellum production. This transition was concurrent with plasmid rearrangements and was stimulated in the presence of wheat seedling exudates. The swarming rate of the Swa(++) derivatives of BK759.G was affected by the local density of their colonies but not by the presence of the exudates.