Shewanella oneidensis MR-1 chemotaxis in a diffusion gradient chamber.

To obtain a systems-level understanding of Shewanella biology and ecology, the influence of electron acceptor availability on Shewanella's growth, metabolism, and transport needs to be elucidated. The diffusion gradient chamber (DGC) is an experimental tool developed to study population-level microbial growth and motility in response to concentration gradients. In this paper, the response of populations of Shewanella oneidensis MR-1 cells to an applied single gradient of the electron acceptor fumarate and applied opposing gradients of fumarate and nitrate, also an electron acceptor, were studied in the DGC. Mathematical models capable of predicting cellular growth and chemotaxis under the influence of gradients were used to analyze the results. Examining wild-type cells grown in a single gradient of fumarate, we found that MR-1 cells formed a chemotactic band that migrated up the electron acceptor gradient essentially as predicted by the model. The predicted velocity of the chemotactic cell band advancing toward the chemoattractant source (0.139 cm/h, R(2) = 0.996) closely matched that measured in the DGC (0.134 cm/h, R(2) = 0.997). Investigating the impact of opposing gradients of nitrate and fumarate on the chemotactic behaviors of S. oneidensis MR-1 fumarate reductase and nitrate reductase mutants, we found that the DGC was able to separate these mutants based upon their abilities to use the available electron acceptors in accordance with model predictions. Differences in the ability of Shewanella species to respond to and use available electron acceptors is thought to play an important role in their ecology. Therefore, these results validate the use of the DGC system to measure and simulate Shewanella chemotaxis in response to electron acceptor gradients and establish it as a research tool to help elucidate Shewanella's role in environmental processes.