Shewanella species are famous for their broad range of terminal electron acceptors and the ability to perform taxis towards both soluble and insoluble electron acceptors, which may help explain their nearly ubiquitous presence in widely disparate environmental niches around the world. Studies of Shewanella's tactic properties are important to understand their competitiveness and roles in elemental (e.g. nitrogen, sulfur, iron, manganese and others) cycling and bioremediation (e.g., precipitation of soluble uranium oxides). Population-level microbial taxis involves a complex interplay of cellular process (e.g., growth, metabolism, chemotaxis, and random motility) and molecular processes (e.g., diffusion of electron donors and acceptors that serve as attractants). This dissertation describes the use of multiple approaches including engineering tools, biological assays and mathematical models to study population-level growth and taxis of Shewanella in response to applied and cell-generated gradients of soluble electron acceptors. The model was able to reproduce key trends of the observed cell growth and migration patterns in either diffusion gradient chamber (DGC) or motility assays, which validate the use of our approaches to measure and simulate Shewanella's taxis in response to electron acceptor gradients.New hypotheses relevant to Shewanella's taxis were investigated to help understand how ecological niches containing various electron acceptor gradients influence the distribution of Shewanella baltica strains with different genotypes and hence different chemotactic behaviors. We studied the impact of opposing gradients of nitrate and fumarate on the chemotactic behaviors of S. oneidensis MR-1 fumarate reductase and nitrate reductase mutants, where the mixture of mutant strains could be partially separated into two populations via formation of two separate chemotactic bands, one moving toward each electron acceptor source. We also studied cell behavior in the presence of insoluble electron acceptor manganese dioxide (MnO2). A novel mechanism, mediated energy taxis, we proposed by which shewanellae use self-secreted riboflavin as both an electron shuttle and an attractant to direct cell movement toward local sources of insoluble electron acceptors. To test this hypothesis, taxis measurements were conducted in the presence of in various insoluble electron acceptors and with chemotaxis mutants. The results strongly supported the hypothesis, and mathematical models based on these mediated energy taxis mechanisms were able to predict experimental trends.
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