OXYGEN TAXIS AND PROTON MOTIVE FORCE IN AZOSPIRILLUM-BRASILENSE

The microaerophilic nitrogen-fixing bacterium Azospirillum brasilense formed a sharply defined band in a spatial gradient of oxygen. As a re sult of aerotaxis, the bacteria were attracted to a specific low conce ntration of oxygen (3 to 5 mu M). Bacteria swimming away from the aero tactic band were repelled by the higher or lower concentration of oxyg en that they encountered and returned to the band. This behavior was c onfirmed by using temporal gradients of oxygen. The cellular energy le vel in A. brasilense, monitored by measuring the proton motive force, was maximal at 3 to 5 9 mu M oxygen. The proton motive force was lower at oxygen concentrations that were higher or lower than the preferred oxygen concentration. Bacteria swimming toward the aerotactic band wo uld experience an increase in the proton motive force, and bacteria sw imming away from the band would experience a decrease in the proton mo tive force. It is proposed that the change in the proton motive force is the signal that regulates positive and negative aerotaxis. The pref erred oxygen concentration for aerotaxis was similar to the preferred oxygen concentration for nitrogen fixation. Aerotaxis is an important adaptive behavioral response that can guide these free-living diazotro phs to the optimal niche for nitrogen fixation in the rhizosphere.