Modeling torque versus speed, shot noise, and rotational diffusion of the bacterial flagellar motor

TL;DR

This paper introduces a minimal physical model of the bacterial flagellar motor that explains its torque-speed relationship, shot noise effects, and rotational diffusion, providing insights into energy transduction and motor fluctuations.

Contribution

The model explains the torque-speed relationship and shot noise effects in bacterial flagellar motors, linking proton flux saturation to torque drop and diffusion.

Findings

Torque drops sharply at high speeds due to proton flux saturation.

Shot noise in proton current dominates rotational diffusion at low loads.

The model offers a new way to probe energy source discreteness in molecular motors.

Abstract

We present a minimal physical model for the flagellar motor that enables bacteria to swim. Our model explains the experimentally measured torque-speed relationship of the proton-driven E. coli motor at various pH and temperature conditions. In particular, the dramatic drop of torque at high rotation speeds (the "knee") is shown to arise from saturation of the proton flux. Moreover, we show that shot noise in the proton current dominates the diffusion of motor rotation at low loads. This suggests a new way to probe the discreteness of the energy source, analogous to measurements of charge quantization in superconducting tunnel junctions.