Molecular and Functional Aspects of Bacterial Chemotaxis

We consider the dynamics of chemotaxis in the model bacterium Escherichia coli. We analyze both its molecular mechanisms and the functional causes governing the evolution of the observed behaviors. We review molecular models of the transduction network controlling the bacterial chemotaxis in response to chemoattractant binding to the receptors. In particular, recent progress stimulated by FRET experiments is presented for statistical physics allosteric models. The response function to a pulse of chemoattractant is expressed in terms of microscopic parameters of the allosteric models. The functional causes for the shape of the response function, as measured in experimental tethering assay, are then investigated. A hydrodynamic equation, valid for space-time scales larger than the microscopic running length and time, is derived for the position of a swimming bacterium. It is then shown how optimization over the microscopic parameters of the response function yields the curve observed experimentally. In particular, the observed property of adaptation to the background level of aspartate emerges as being produced by fluctuations in the space-time chemoattractant profiles sensed by bacteria along their trajectories. This functional cause is distinct from arguments based on the extension of the dynamical range. Future directions and experiments to probe the adaptation of E. coli chemotaxis to the environmental conditions and its response to realistic space-time chemoattractant stimuli are finally discussed.