Immobility of isolated swarmer cells due to local liquid depletion

TL;DR

This study investigates why isolated swarmer bacterial cells become immobile at low densities, revealing that local liquid depletion and flagella state are key factors, challenging simplified dry models of active matter.

Contribution

It uncovers the biophysical mechanisms behind bacterial immobility at low densities, emphasizing the role of liquid depletion and flagella configuration, which were previously not well understood.

Findings

Immobilization correlates with local liquid depletion.

Flagella are spread out in liquid-depleted regions.

Dry models are insufficient to describe swarming behavior.

Abstract

Bacterial swarming is a complex phenomenon in which thousands of self-propelled rod-shaped cells move coherently on surfaces, providing an excellent example of active matter. However, bacterial swarming is different from most studied examples of active systems because single isolated cells do not move, while clusters do. The biophysical aspects underlying this behavior are unclear. In this work we explore the case of low local cell densities, where single cells become temporarily immobile. We show that immobility is related to local depletion of liquid. In addition, it is also associated with the state of the flagella. Specifically, the flagellar bundles at (temporarily) liquid depleted regions are completely spread-out. Our results suggest that dry models of self-propelled agents, which only consider steric alignments and neglect hydrodynamic effects, are oversimplified and are not sufficient to describe swarming bacteria.