Chemotaxis of Azospirillum lipoferum Sp 59b and Azospirillum brasilense Sp 7 and Sp CD to malate and to the aromatic substrates benzoate, protocatechuate, 4-hydroxybenzoate, and catechol was assayed by the capillary method and direct cell counts. A. lipoferum required induction by growth on 4-hydroxybenzoate for positive chemotaxis to this compound. Chemotaxis of Azospirillum spp. to all other substrates did not require induction. Maximum chemotactic responses for most aromatic compounds occurred at concentrations of 1 to 10 mM for A. lipoferum and 100 μM to 1 mM for A. brasilense. Threshold levels of these chemoattractants ranged from nanomolar to micromolar, with A. brasilense Sp CD showing the lowest threshold levels for the substrates tested. Benzoate was the strongest chemoattractant tested, with threshold concentrations in the nanomolar to picomolar range for all strains. Azospirillum spp. clearly have more sensitive chemosensory mechanisms for certain aromatic substrates than previously reported in some other soil bacteria. This sensitivity allows Azospirillum spp. to detect and respond to aromatic substrates at concentrations relevant to the soil and rhizosphere environments. The ability to detect such low concentrations of aromatic compounds in soils may confer advantages in survival and colonization of the rhizosphere by Azospirillum species
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