The chemotaxis of Escherichia coli (E. coli) and other bacteria in real environments is subject to noise from outside, in addition to that within the cells. While the cells have internal mechanisms to filter intracellular noise, external intervention is needed for noise from the environment. In previous microbial applications, neural networks have been more effective in filtering environmental noise than algorithmic devices. On this basis, a number of neural network configurations are explored for their effectiveness as external filters in consonance with internal filtering in the chemotaxis of E. coli. Simulations showed significant improvements with neural filters, with the auto-associative filter being the best. The results also revealed the possibility of stochastic resonance between the filtered internal and external noise being a cause of such improvement, and this likelihood is supported by previous reports in different applications.

Noise is a pervasive feature of many microbial processes. Cells moving in a real environment, as contrasted with controlled laboratory conditions, experience more than one source of noise, some present within the cells (Rao et al., 2002), some emanating from the extracellular environment (Patnaik, 2006a), that is, the noise associated with the binding process between the cells and chemicals from the environment.

The third source of noise is significant in the chemotaxis of bacterial and other cells. This study concentrates on bacterial chemotaxis not because the chemotaxis of other types of cells is unaffected by binding noise, but because their mechanisms and responses are different (Levchenko and Iglesias, 2002; and Andrews et al., 2006). Studies of bacterial chemotaxis have generally employed Escherichia coli (E. coli) as a model system because of its well-understood biochemistry, the simplicity of its chemosensory network and its usefulness as a host for genes from other cells.

Chemotaxis refers to the movement of cells in response to chemical stimuli. If the stimulus is favorable, the cells move toward it; similarly, they move away from unpleasant or hostile stimuli. The former phenomenon is chemoattraction, and it forms a major fraction of useful chemotactic processes. To decide their directions of movement, bacteria should sense the gradient of a chemoattractant. This is done by comparing the concentration of the chemoattractant at two points, either on the cell surface, or across a span of time. Since E. coli are small, spatial differences across a cell are not significant. The cells therefore determine a favorable direction by comparing the chemoattractant concentration as they move along (Baker et al., 2006).

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