Filling an emulsion drop with motile bacteria

TL;DR

This study investigates how motile bacteria distribute within water droplets in oil, revealing surface accumulation at low concentrations and uniform filling at higher densities, supported by simulations and theoretical models.

Contribution

It provides new insights into bacterial spatial distribution inside emulsified droplets, combining experimental measurements with simulations and theoretical analysis.

Findings

Cell density peaks at the water-oil interface at low concentrations.

Bulk density becomes uniform as cell concentration increases.

Simulation and theory explain the transition based on cell traffic and scattering.

Abstract

We have measured the spatial distribution of motile Escherichia coli inside spherical water droplets emulsified in oil. At low cell concentrations, the cell density peaks at the water-oil interface; at increasing concentration, the bulk of each droplet fills up uniformly while the surface peak remains. Simulations and theory show that the bulk density results from a `traffic' of cells leaving the surface layer, increasingly due to cell-cell scattering as the surface coverage rises above $\sim 10\%$. Our findings show similarities with the physics of a rarefied gas in a spherical cavity with attractive walls.