Bacteria solve the problem of crowding by moving slowly

TL;DR

This study reveals that bacteria like Pseudomonas aeruginosa use slow movement to avoid topological defect trapping, enabling efficient collective migration in dense environments, influenced by liquid crystal physics.

Contribution

It uncovers how liquid crystal defect physics explains bacterial crowding solutions, highlighting the evolutionary advantage of slower motility in dense groups.

Findings

Wild-type bacteria outcompete hyperpilated mutants at high density.

Topological defects cause fast cells to become trapped and rotate vertically.

Slower movement helps bacteria avoid defect trapping and enhances collective migration.

Abstract

In systems as diverse as migrating mammals to road traffic, crowding acts to inhibit efficient collective movement. Bacteria, however, are observed to move in very dense groups containing billions of individuals without causing the gridlock common to other systems. Here we combine experiments, cell tracking and individual-based modelling to study the pathogen Pseudomonas aeruginosa as it collectively migrates across surfaces using grappling-hook like pili. We show that the fast moving cells of a hyperpilated mutant are overtaken and outcompeted by the slower moving wild-type at high cell densities. Using theory developed to study liquid crystals, we demonstrate that this effect is mediated by the physics of topological defects, points where cells with different orientations meet one another. Our analyses reveal that when comet-like defects collide with one another, the fast-moving mutant cells rotate vertically and become trapped. By moving more slowly, wild-type cells avoid this trapping mechanism, allowing them to collectively migrate faster. Our work suggests that the physics of liquid crystals has played a pivotal role in the evolution of collective bacterial motility by exerting a strong selection for cells that exercise restraint in their movement.