Diffusive transport without detailed balance in motile bacteria: Does microbiology need statistical physics?

TL;DR

This paper reviews how bacterial motility, driven by chemical energy, leads to diffusive behavior without detailed balance, highlighting the relevance of statistical physics in modeling such biological systems.

Contribution

It introduces statistical mechanical models for bacterial motility as a paradigm for understanding diffusion without detailed balance in microbiology.

Findings

Motility parameters depend on local environment and vary in space.

Steady states in motile bacteria are not time-reversal invariant.

Statistical physics offers useful insights into bacterial diffusion phenomena.

Abstract

Microbiology is the science of microbes, particularly bacteria. Many bacteria are motile: they are capable of self-propulsion. Among these, a significant class execute so-called run-and-tumble motion: they follow a fairly straight path for a certain distance, then abruptly change direction before repeating the process. This dynamics has something in common with Brownian motion (it is diffusive at large scales), and also something in contrast. Specifically, motility parameters such as the run speed and tumble rate depend on the local environment and hence can vary in space. When they do so, even if a steady state is reached, this is not generally invariant under time-reversal: the principle of detailed balance, which restores the microscopic time-reversal symmetry of systems in thermal equilibrium, is mesoscopically absent in motile bacteria. This lack of detailed balance (allowed by the flux of chemical energy that drives motility) creates pitfalls for the unwary modeller. Here I review some statistical mechanical models for bacterial motility, presenting them as a paradigm for exploring diffusion without detailed balance. I also discuss the extent to which statistical physics is useful in understanding real or potential microbiological experiments.