Bacterial chemotaxis is a key survival strategy in diverse environments. It is also an important behavior that allows motile bacteria to colonize new niches. Azospirillum brasilense are motile diazotrophic bacteria of agricultural interests due to the ability of several strains to promote growth of a variety of plants upon inoculation. The genome of A. brasilense is predicted to encode four chemotaxis pathways, two of which (Che2 and Che3) do not control the chemotaxis response. The chemotaxis system, named Che1, was shown in previous work to regulate transient changes in swimming velocity that occur during chemotaxis. However, Che1 had a minor role in controlling changes in the probability of reversals in the direction of swimming which are also hallmark of the chemotaxis response of motile A. brasilense cells. In this dissertation, using genetic and behavioral assays, we demonstrate that the Che4 chemotaxis system regulates the probability of swimming reversals and is the major signaling pathway for chemotaxis and wheat root surface colonization in A. brasilense. We also showed that Che1 and Che4 function together to coordinate changes in the swimming motility pattern and that the effect of Che1 on swimming speed functions to enhance the chemotactic response. In the latter half of this dissertation, we focused on the motility and the role of different CheY homologs in chemotaxis and motility of A. brasilense. We used high throughput single cell tracking to analyze the swimming pattern of motile A. brasilense and identified three different swimming patterns: run-reverse, run-pause and run-reverse-flick like pattern. We also showed that different CheY homologs differently affect the probability of transient pauses during swimming and obtain evidence that the transient pauses are controlled by chemotaxis signaling. These diverse swimming patterns may be advantageous to navigate the heterogeneous and porous environment of the soil. Collectively, our findings illustrate novel mechanisms by which motile bacteria utilize two chemotaxis systems to regulate speed and reversal frequency, and transient pauses during swimming to enhance chemotaxis
