Exact model of aerotactic band: From Fokker-Planck equation to band structure and fluid flow

TL;DR

This paper develops an exact microscopic model of bacterial aerotactic bands using solutions to the Fokker-Planck equation, linking oxygen consumption, bacterial response, and fluid flow, advancing understanding of self-organization in bacteria.

Contribution

It provides the first analytical solutions connecting oxygen dynamics, bacterial behavior, and fluid flow in aerotactic bands, extending beyond previous mesoscopic or numerical models.

Findings

Derived a third-order nonlinear differential equation for oxygen and bacterial response.

Analytical solutions for two aerotactic behaviors and resulting band structures.

Predicted fluid flow induced by bacterial bands consistent with experimental observations.

Abstract

A variety of bacterial species spontaneously assemble in aerotactic band, local accumulation at a fixed distance from the air-water interface. Although the phenomenon is long known, its modelling is so far limited to mesoscopic, one-dimensional or numerical descriptions. We investigate band properties at the microscopic scale using exact solutions to the Fokker-Planck equation. First, we show that the interplay between oxygen consumption and tumbling modulation is governed by a third-order nonlinear differential equation relating the oxygen concentration to the aerotactic response. For two model aerotactic behaviors, we present analytical solutions and discuss the resulting band structure. Second, we investigate how an aerotactic band of magnetotactic bacteria in a magnetic field induces a spontaneous fluid flow, as observed in experiments [Marmol et al, arXiv 2025]. In the low field limit, we determine the bacterial distribution and the active stress tensor. Using the Green function of the hydrodynamic problem, we obtain a prediction for the fluid flow that is both simple and consistent with observations. Altogether, our results provide a model system of aerotactic band and solid ground to analyze aerotaxis-driven self-organization.