Discovery of oscillations in rotational speed of body-tethered Caulobacter crescentus

Swarmer cells of Caulobacter crescentus have been found to tether to glass at a point on the cell body. The rolling of the freely rotating flagellum near the glass surface causes the cell body to rotate. We describe the discovery of damped oscillations in the rotational speed of these cell bodies. We show that the damped oscillations are robust over multiple cells and that they depend more on the cell's accumulated rotation angle than on time. We also find that their phase is determined by the moment the flagellar motor changes the direction of its rotation. The oscillations occur only for one direction of cell rotation, when the flagellum is in pulling mode. We discuss possible explanations for these oscillations, including fluctuations in flagellar motor torque and periodic changes in flagellar orientation, and illustrate both of these cases using simplified computer models. Finally, we present the hypothesis that the oscillations are the result of fluctuations in the proton motive force, initiated by a sudden change in proton current that occurs when the motor switches rotation direction.