Aerotactic response of Azospirillum brasilense

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Abstract
Five strains of A. brasilense and 2 of Azospirillum spp., from Israel, responded to self-created and performed O2 gradients by forming aerotactic bands in capillary tubes and actively moving toward a specific zone with low dissolved O2. Increasing the O2 concentration in capillaries containing phosphate buffer increased the number of attracted bacteria and decreased band velocity. High O2 concentrations and H2O2 temporarily repulsed the bacteria, causing the formation of a bacterial arc around the capillary mouth. There was no band formation under anaerobic conditions, although the bacteria remained highly motile. Exogenous energy sources were unnecessary for aerotaxis in Azospirillum spp. The addition of oxidizable substrates to the capillary slightly enhanced aerotaxis, possibly by accelerating O2 consumption. Aerotactic band formation was affected by pH, bacterial concentration and age, incubation time and respiratory inhibitors, but not by the lack of combined N in the growth medium. Aerotaxis apparently plays a role in the capacity of Azospirillum spp. to reach an environment suitable for N2 fixation.