An Element of Determinism in a Stochastic Flagellar Motor Switch

TL;DR

This study reveals that the flagellar motor switch in Vibrio alginolyticus exhibits non-Poissonian, deterministic-like behavior with long-lasting inhibition periods, challenging the assumption of purely stochastic switching in bacterial motility.

Contribution

It introduces a first-passage model based on motor conformation fluctuations to explain the non-Poissonian switching intervals observed in Vibrio alginolyticus.

Findings

Steady-state interval distributions are strongly peaked at finite times.

Motor reversal dynamics are well described by a first-passage process.

Inhibition periods last several hundred milliseconds, indicating deterministic elements.

Abstract

Marine bacterium Vibrio alginolyticus uses a single polar flagellum to navigate in an aqueous environment. Similar to Escherichia coli cells, the polar flagellar motor has two states; when the motor is counter-clockwise, the cell swims forward and when the motor is clockwise, the cell swims backward. V. alginolyticus also incorporates a direction randomization step at the start of the forward swimming interval by flicking its flagellum. To gain an understanding on how the polar flagellar motor switch is regulated, distributions of the forward $\Delta_{f}$ and backward $\Delta_{b}$ intervals are investigated herein. We found that the steady-state probability density functions, $P(\Delta_{f})$ and $P(\Delta_{b})$, of freely swimming bacteria are strongly peaked at a finite time, suggesting that the motor switch is not Poissonian. The short-time inhibition is sufficiently strong and long lasting, i.e., several hundred milliseconds for both intervals, which is readily observed and characterized. Treating motor reversal dynamics as a first-passage problem, which results from conformation fluctuations of the motor switch, we calculated $P(\Delta_{f})$ and $P(\Delta_{b})$ and found good agreement with the measurements.