Bacteria commonly live attached to surfaces in dense collectives containing billions of cells¹. While it is known that motility allows these groups to expand en masse into new territory^2,3,4,5, how bacteria collectively move across surfaces under such tightly packed conditions remains poorly understood. 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^3,6,7. 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^{8,9,10,11,12,13}, 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 defects with topological charge +1/2 collide with one another, the fast-moving mutant cells rotate to point vertically and become trapped. By moving more slowly, wild-type cells avoid this trapping mechanism and generate collective behaviour that results in faster migration. In this way, the physics of liquid crystals explains how slow bacteria can outcompete faster cells in the race for new territory.

细菌通常生活在附有数十亿个细胞^1的密集集合体的表面上。虽然众所周知，运动性使这些群体能够大规模地扩展到新的领域^2,3,4,5，但在如此紧密包装的条件下细菌如何在整个表面上集体移动仍然知之甚少。在这里，我们结合实验，细胞跟踪和基于个体的模型来研究铜绿假单胞菌病原体，因为它使用钩状菌毛^3,6,7共同在整个表面上迁移。我们显示，在高细胞密度下，超缓慢突变型的快速移动细胞被慢速移动的野生型所超越。利用理论研究液晶^{8,9,10,11,12,13}，我们证明了这种效应是由拓扑缺陷的物理机制所介导的，即具有不同方向的细胞彼此相遇的点。我们的分析表明，当拓扑电荷+1/2的缺陷相互碰撞时，快速移动的突变细胞旋转以指向垂直方向并被捕获。通过缓慢移动，野生型细胞避免了这种诱捕机制，并产生了导致更快迁移的集体行为。以此方式，液晶的物理学解释了慢细菌如何在新领域的竞争中胜过更快的细胞。