Fluctuations in Bacterial Suspensions Driven by Chemotaxis

Abstract

Stochastic fluctuations in living systems create a spectacular variety of unique states far from equilibrium, which however remains largely unexplored regarding its underlying physics, especially when considering their instinctive responses to environmental stimuli. Here, by utilizing a molecular-dynamics model of bacterial chemotaxis, we present an investigation of tracer statistics in suspensions of chemotactic bacteria. Unlike Brownian motion in conventional media, the tracer particle performs a short-time ballistic but long-time Fickian diffusion with non-Gaussian dynamics. A phenomenological extension of Langevin equation accounts for the observed anomalous behaviors. Moreover, a violation of the Stokes-Einstein relation is identified regarding the size-dependence of particle diffusivity in bacterial suspensions. Our findings uncover the physical nature of stimulus-driven fluctuation statistics in bacterial fluids, and suggest a theoretical framework to deepen the understanding of the nonequilibrium statistical physics of active matter under external stimuli.