Chemoataxis in colloidal and bacterial systems

Institute of Theoretical Physics

Technical University of Berlin

Chemoataxis can be defined as the motion along chemical gradients. We study its effects in a synthetic system of active Janus particles and its particular properties for the bacterium Escherichia coli. Recent experiments of active colloids show a coexistence of single particles and clusters. In such a "dynamic clustering state" the clusters are themselves active and their sizes vary in time. Experiments of self propelling bacteria show similar behavior, however, here a collapse to aggregates with large density is observed. We present a model of mobile colloids which exhibits both: a dynamic state and a collapsed state. Each particle produces a gradient of a chemical which generates phoretic motion of nearby colloids leading to an e ective chemotactic interaction. We distinguish between translative and oriented chemotaxis and four different cluster states including collapse and dynamic clustering. Chemotaxis of the bacterium Escherichia coli is based on an intracellular response mechanism to chemoattractans or repellents. It moves with alternating states of runs and tumbles that occur with a mean tumble rate which is reduced when moving up a gradient of a chemoattractant. We set up a time-continuous model that describes runs and tumbles as a stochastic process including shot noise. It reproduces E.Coli's motion quantitatively. In a next step, we introduce adequate Kramer-Moyal-coeffcients which can be analytically determined in our model. Matching them with the corresponding moments gained from experimental trajectories gives insight in the tumbling and chemotaxis behavior without making use of complicated tumble recognition procedures.