Using a digital tracking microscope that provides both cell position and orientation, we have correlated the detailed motion of the cell body of a fast-swimming bacterium, Caulobacter crescentus, with its swimming motility. Contrary to the prevailing view that the rotating flagellum is the only means to propel the cell, we show that when the flagellum pushes the cell, the axis of the cell body precesses with a helical trajectory that enhances motility. This discovery changes our understanding regarding the role that cell shape and motion plays in bacterial motility. Furthermore, our powerful cell tracking technique enables a wide variety of studies that require extended observation of single cells, including motility, cell behavior, and aging.

Abstract

We resolve the 3D trajectory and the orientation of individual cells for extended times, using a digital tracking technique combined with 3D reconstructions. We have used this technique to study the motility of the uniflagellated bacterium Caulobacter crescentus and have found that each cell displays two distinct modes of motility, depending on the sense of rotation of the flagellar motor. In the forward mode, when the flagellum pushes the cell, the cell body is tilted with respect to the direction of motion, and it precesses, tracing out a helical trajectory. In the reverse mode, when the flagellum pulls the cell, the precession is smaller and the cell has a lower translation distance per rotation period and thus a lower motility. Using resistive force theory, we show how the helical motion of the cell body generates thrust and can explain the direction-dependent changes in swimming motility. The source of the cell body precession is believed to be associated with the flexibility of the hook that connects the flagellum to the cell body.