A phase diagram for bacterial swarming

TL;DR

This paper explores the physical principles of bacterial swarming, revealing a phase diagram of different collective behaviors influenced by cell shape and density, highlighting how bacteria optimize their swarming capabilities.

Contribution

It provides the first detailed phase diagram of bacterial swarming regimes based on cell shape and density, linking physical properties to collective dynamics.

Findings

Distinct swarming regimes depend on cell aspect ratio and density

Standard conditions promote rapid mixing and homogeneous density

Elongated mutants exhibit clustering behavior

Abstract

Bacterial swarming is a rapid mass-migration, in which thousands of cells spread collectively to colonize a surface. Physically, swarming is a natural example of active particles that use energy to generate motion. Accordingly, understanding the constraints physics imposes on the dynamics is essential to understand the mechanisms underlying the swarming phenomenon. We present new experiments of swarming Bacillus subtilis mutants with different aspect ratios and densities. Analyzing the dynamics reveals a rich phase diagram of qualitatively distinct swarming regimes, describing how the shape and density of cells govern the global dynamical characteristics of the entire swarm. Moreover, we show that under standard conditions bacteria inhabit a region of phase space that is associated with rapid mixing and robust dynamics, with homogeneous density and no preferred direction of motion. This contrasts characteristic clustering behavior of self-propelled rods that is recovered only for very elongated mutant species. Thus, bacteria have adapted their physics to optimize the principle functions assumed for swarming.