Motility Modulates the Partitioning of Bacteria in Aqueous Two-Phase Systems

TL;DR

This study investigates how bacterial motility influences their distribution in aqueous two-phase systems, revealing that motile bacteria can cross phase boundaries due to a balance of mechanical forces and interfacial interactions.

Contribution

It demonstrates that bacterial motility affects phase partitioning in two-phase systems, introducing a model of active rods overcoming soft confinement at interfaces.

Findings

Motile bacteria penetrate the DEX/PEG interface unlike non-motile bacteria.

Partitioning of motile bacteria varies with DEX composition.

Mechanical forces from motility can overcome interfacial confinement.

Abstract

We study the partitioning of motile bacteria in an aqueous two-phase mixture of dextran (DEX) and polyethylene glycol (PEG), which can phase separate into DEX-rich and PEG-rich phases. While non-motile bacteria partition exclusively into the DEX-rich phase in all conditions tested, we observed that motile bacteria penetrate the soft DEX/PEG interface and partition variably among the two phases. For our model organism \textit{Bacillus subtilis}, the fraction of motile bacteria in the DEX-rich phase increased from 0.58 to 1 as we increased DEX composition within the two-phase region. We hypothesized that the chemical affinity between DEX and the bacteria cell wall acts to weakly confine the bacteria within the DEX-rich phase; however, motility can generate sufficient mechanical forces to overcome the soft confinement and propel the bacteria into the PEG-rich phase. Using optical tweezers to drag a bacterium across the DEX/PEG interface, we demonstrate that the overall bacteria partitioning is determined by a competition between the interfacial forces and bacterial propulsive forces. Our measurements are supported by a theoretical model of dilute active rods embedded within a periodic soft confinement potential.