Enhanced diffusion of nonswimmers in a three-dimensional bath of motile bacteria

TL;DR

This study demonstrates that non-motile cells in a 3D bacterial population experience increased diffusivity due to active bacteria, with enhancement proportional to bacterial flux, primarily driven by fluid entrainment effects.

Contribution

It reveals that fluid entrainment, not hydrodynamic radius differences, causes diffusion enhancement of non-motile cells in 3D bacterial suspensions.

Findings

Diffusivity of non-motile cells increases with active cell flux.

Hydrodynamic radius differences do not affect diffusivity enhancement.

Fluid entrainment explains the diffusion increase within 2% accuracy.

Abstract

We show, using differential dynamic microscopy, that the diffusivity of non-motile cells in a three-dimensional (3D) population of motile E. coli is enhanced by an amount proportional to the active cell flux. While non-motile mutants without flagella and mutants with paralysed flagella have quite different thermal diffusivities and therefore hydrodynamic radii, their diffusivities are enhanced to the same extent by swimmers in the regime of cell densities explored here. Integrating the advective motion of non-swimmers caused by swimmers with finite persistence-length trajectories predicts our observations to within 2%, indicating that fluid entrainment is not relevant for diffusion enhancement in 3D.