The Fluid Mechanics of Bacterial Motility

Many bacteria have evolved a motility system that consists of a helical appendage attached to the cell body by a rotary molecular motor. The motor is driven by protons flowing from the outside to the inside of the cell. The resulting rotation of the helix propels the bacterium through its low Reynolds number fluid environment. In this work, we determine the energy cost of bacterial motility by inputting experimentally measured bacterial motion into computational fluid dynamics simulations. The computational method is precisely calibrated using macroscopic fluid dynamics experiments to ensure the calculated energy values are accurate. We compute the energy cost of motility over a range of body and helix geometries, and we find that the geometries realized by living bacteria are near the minimum value of this measure.