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Optimal Chemotactic Responses in Stochastic Environments

Most of our understanding of bacterial chemotaxis comes from studies of Escherichia coli. However, recent evidence suggests significant departures from the E. coli paradigm in other bacterial species. This variation may stem from different species inhabiting distinct environments and thus adapting to specific environmental pressures. In particular, these complex and dynamic environments may be poorly represented by standard experimental and theoretical models. In this work, we study the performance of various chemotactic strategies under a range of stochastic time- and space-varying attractant distributions in silico. We describe a novel type of response in which the bacterium tumbles more when attractant concentration is increasing, in contrast to the response of E. coli, and demonstrate how this response explains the behavior of aerobically-grown Rhodobacter sphaeroides. In this "speculator" response, bacteria compare the current attractant concentration to the long-term average. By tumbling persistently when the current concentration is higher than the average, bacteria maintain their position in regions of high attractant concentration. If the current concentration is lower than the average, or is declining, bacteria swim away in search of more favorable conditions. When the attractant distribution is spatially complex but slowly-changing, this response is as effective as that of E. coli.