Steps in the bacterial flagellar motor

TL;DR

This paper presents a physical model explaining the stepping behavior of the bacterial flagellar motor as a random walk in a tilted potential, accounting for experimental observations and predicting new behaviors like sublinear torque-speed relation.

Contribution

It introduces a simple physical model that explains the stepping mechanism and properties of the bacterial flagellar motor based on angular position and contact forces.

Findings

Model accounts for step size differences between forward and backward steps.

Predicts a sublinear torque-speed relationship at low torque.

Forecasts a peak in rotor diffusion as a function of torque.

Abstract

The bacterial flagellar motor is a highly efficient rotary machine used by many bacteria to propel themselves. It has recently been shown that at low speeds its rotation proceeds in steps [Sowa et al. (2005) Nature 437, 916--919]. Here we propose a simple physical model that accounts for this stepping behavior as a random walk in a tilted corrugated potential that combines torque and contact forces. We argue that the absolute angular position of the rotor is crucial for understanding step properties, and show this hypothesis to be consistent with the available data, in particular the observation that backward steps are smaller on average than forward steps. Our model also predicts a sublinear torque-speed relationship at low torque, and a peak in rotor diffusion as a function of torque.