The Limiting Speed of the Bacterial Flagellar Motor

TL;DR

This paper challenges the idea of a universal limiting speed for bacterial flagellar motors, proposing that the maximum speed increases with more torque-generating units due to stator disengagement at low loads.

Contribution

It introduces a new model suggesting the maximum speed depends on the number of active stators, contrasting previous assumptions of universality.

Findings

Maximum speed increases with additional stators.

Stators disengage at low loads, affecting speed.

High duty ratio at high loads supports the model.

Abstract

Recent experiments on the bacterial flagellar motor have shown that the structure of this nanomachine, which drives locomotion in a wide range of bacterial species, is more dynamic than previously believed. Specifically, the number of active torque-generating units (stators) was shown to vary across applied loads. This finding invalidates the experimental evidence reporting that limiting (zero-torque) speed is independent of the number of active stators. Here, we propose that, contrary to previous assumptions, the maximum speed of the motor is not universal, but rather increases as additional torque-generators are recruited. This result arises from our assumption that stators disengage from the motor for a significant portion of their mechanochemical cycles at low loads. We show that this assumption is consistent with current experimental evidence and consolidate our predictions with arguments that a processive motor must have a high duty ratio at high loads.