Active Contact Forces Drive Nonequilibrium Fluctuations in Membrane Vesicles.

Area of Science:

• Biophysics
• Soft Matter Physics
• Cellular Biophysics

Background:

• Giant unilamellar vesicles (GUVs) are model systems for cell membranes.
• Membrane fluctuations are typically governed by thermal energy and the fluctuation-dissipation theorem.
• Motile bacteria introduce active, nonequilibrium forces into biological systems.

Purpose of the Study:

• To investigate the impact of encapsulated motile bacteria on the shape fluctuations of GUVs.
• To quantify the deviation from equilibrium thermal fluctuations.
• To understand the underlying mechanisms driving these nonequilibrium phenomena.

Main Methods:

• Experimental observation of GUV shape fluctuations with encapsulated bacteria.
• Numerical simulations using a modified Langevin equation incorporating bacteria-membrane forces.
• Theoretical analysis using dynamical membrane equations and fluctuation-dissipation principles.

Main Results:

• A significant increase in membrane fluctuation magnitude at low wave numbers was observed, exceeding thermal levels.
• Simulations and theory showed excellent agreement with experimental data.
• The fluctuation-dissipation theorem was found to hold, with fluctuations decomposable into thermal and active components.

Conclusions:

• Motile bacteria actively drive GUV membrane shape fluctuations beyond thermal equilibrium.
• Bacteria-induced forces contribute an active component to membrane dynamics.
• The study provides a framework for understanding active matter interactions with membranes.