Physical characteristics of mixed-species swarming colonies

TL;DR

This study investigates the physical properties and dynamics of mixed-species bacterial swarms, revealing how species interactions influence collective behavior and colony structure in Bacillus subtilis and Serratia marcescens.

Contribution

First experimental analysis of mixed-species swarming behavior, showing how species ratios affect physical and dynamical properties of bacterial colonies.

Findings

Mixed-species swarms grow faster than single-species cultures.

Species ratio determines vortex size and colony structure.

Mixed populations maintain coordinated swarming behavior.

Abstract

In nature, bacterial collectives typically consist of multiple species, which are interacting both biochemically and physically. Nonetheless, past studies on the physical properties of swarming bacteria were focused on axenic (single species) populations. In bacterial swarming, intricate interactions between the individuals lead to clusters, rapid jets and vortices that depend on cell characteristics such as speed and length. In this work, we show the first results of rapidly swarming mixed-species populations of {\sl Bacillus subtilis} and {\sl Serratia marcescens}, two model swarm species that are known to swarm well in axenic situations. In mixed liquid cultures, both species have higher reproduction rates. We show that the mixed population swarms together well and that the fraction between the species determines all dynamical scales - from the microscopic (e.g. speed distribution), mesoscopic (vortex size) and macroscopic (colony size structure). Understanding mixed-species swarms is essential for a comprehensive understanding of the bacterial swarming phenomenon and its biological and evolutionary implications.