Heterogeneous bacterial swarms with mixed lengths

TL;DR

This study investigates mixed bacterial swarms with different cell lengths, revealing that heterogeneity influences local clustering and swarm speed without causing phase segregation, highlighting a potential advantage of diversity.

Contribution

The paper provides experimental and simulation evidence that heterogeneity in bacterial cell lengths affects swarm dynamics and reveals new insights into collective behavior without phase separation.

Findings

Long cells act as nucleation sites for clusters.

Heterogeneity enhances local swarm speed.

High fractions of long cells hinder overall swarming efficiency.

Abstract

Heterogeneous systems of active matter exhibit a range of complex emergent dynamical patterns. In particular, it is difficult to predict the properties of the mixed system based on its constituents. These considerations are particularly significant for understanding realistic bacterial swarms, which typically develop heterogeneities even when grown from a single cell. Here, mixed swarms of cells with different aspect ratios are studied both experimentally and in simulations. In contrast with previous theory, there is no macroscopic phase segregation. However, locally, long cells act as nucleation cites, around which ordered clusters of short, rapidly moving cells can form, resulting in enhanced swarming speeds. On the other hand, high fractions of long cells form a bottle-neck for efficient swarming. Our results suggest a new physical advantage for the spontaneous heterogeneity of bacterial swarm populations.