Wetting-coupled phase separation as an energetic mechanism for active bacterial adhesion

TL;DR

This study reveals that wetting-induced liquid-liquid phase separation acts as an energetic mechanism for bacterial adhesion, with bacterial motility influencing accumulation and detachment in complex fluids.

Contribution

It introduces wetting-coupled LLPS as a universal physical mechanism for bacterial adhesion, combining experiments and simulations to explain interfacial organization in active suspensions.

Findings

Wetting-coupled LLPS stabilizes bacterial adhesion via interfacial free-energy minimization.

Bacterial motility enhances accumulation at low phase volumes and causes detachment at high volumes.

Capillary interactions induce lateral clustering of bacteria on wetting phases.

Abstract

The rapid adhesion of motile bacteria from dilute suspensions poses a fundamental non-equilibrium problem: hydrodynamic interactions bias bacterial motion near surfaces without generating stable confinement, while electrostatic interactions are predominantly repulsive. Here, combining experiments on Pseudomonas aeruginosa and Staphylococcus aureus in a polyethylene glycol/dextran aqueous two-phase system with large-scale hydrodynamic simulations, we identify wetting-coupled liquid--liquid phase separation (LLPS) as an energetic trapping mechanism for bacterial adhesion. When bacteria partition into a phase that preferentially wets the substrate, interfacial free-energy minimization creates a deep energetic trap that stabilizes adhesion and induces lateral clustering via capillary interactions. Crucially, bacterial motility plays a dual role: at low phase volume fractions, activity enhances transport into the wetting layer and promotes accumulation, whereas at higher phase volumes it suppresses adhesion through the formation of self-spinning droplets that generate hydrodynamic lift opposing interfacial trapping. Our results establish wetting-coupled LLPS as a generic physical route governing interfacial organization in active suspensions. This provides a unified energetic framework for bacterial adhesion in complex fluids, with broad implications for deciphering bacterial-cell interactions and controlling biofilm formation.