Random motility of swimming bacteria: Single cells compared to cell populations

The motility of a population of swimming bacteria can be characterized by a random motility coefficient, μ, the operational equivalent of a diffusion coefficient at the macroscopic level and in the absence of interacting chemical gradients. At the microscopic level, random motility is related to the single‐cell parameters: speed, tumbling probability, and index of directional persistence (related to the angle a cell's path assumes following a change in direction). Various mathematical models have been proposed for relating the macroscopic random motility coefficient to these microscopic single‐cell parameters. In separate experiments, we have measured motility at both the cell‐population and single‐cell levels for Escherichia coli. The agreement of these results shows that the macroscopic transport behavior of a population of motile bacteria can be predicted from straightforward microscopic observations on single cells.