Differential Dynamic Microscopy of Bacterial Motility

We demonstrate a method for the fast, high-throughput characterization of the dynamics of active particles. Specifically, we measure the swimming speed distribution and motile cell fraction in Escherichia coli suspensions. By averaging over similar to 10(4) cells, our method is highly accurate compared to conventional tracking, yielding a routine tool for motility characterization. We find that the diffusivity of nonmotile cells is enhanced in proportion to the concentration of motile cells.