Films of bacteria at interfaces: three stages of behaviour

TL;DR

This study investigates the dynamic formation and mechanical properties of bacterial films at oil-water interfaces, revealing their evolution from active bacterial motion to viscoelastic and elastic behaviors over time.

Contribution

It provides a comprehensive analysis of the three stages of bacterial film development and characterizes their mechanical properties using microrheology and elastometry.

Findings

Bacteria induce superdiffusive probe motion initially.

Bacterial films become viscoelastic and glassy over minutes.

Films exhibit elastic moduli similar to bacteria and show hysteresis at large strains.

Abstract

Bacterial attachment to a fluid interface can lead to the formation of a film with physicochemical properties that evolve with time. We study the time evolution of interface (micro)mechanics for interfaces between oil and bacterial suspensions by following the motion of colloidal probes trapped by capillarity to determine the interface microrheology. Initially, active bacteria at and near the interface drive superdiffusive motion of the colloidal probes. Over timescales of minutes, the bacteria form a viscoelastic film which we discuss as a quasi-two-dimensional, active, glassy system. To study late stage mechanics of the film, we use pendant drop elastometry. The films, grown over tens of hours on oil drops, are expanded and compressed by changing the drop volume. For small strains, by modeling the films as 2D Hookean solids, we estimate the film elastic moduli, finding values similar to those reported in the literature for the bacteria themselves. For large strains, the films are highly hysteretic. Finally, from wrinkles formed on highly compressed drops, we estimate film bending energies. The dramatic restructuring of the interface by such robust films has broad implications, e.g. in the study of active colloids, in understanding the community dynamics of bacteria, and in applied settings including bioremediation.