The soil bacterium Myxococcus xanthus lives in densely packed groups that form dynamic three-dimensional patterns in response to environmental changes, such as droplet-like fruiting bodies during starvation¹. The development of these multicellular structures begins with the sequential formation of cell layers in a process that is poorly understood². Here, using confocal three-dimensional imaging, we find that motile, rod-shaped M. xanthus cells are densely packed and aligned in each layer, forming an active nematic liquid crystal. Cell alignment is nearly perfect throughout the population except at point defects that carry half-integer topological charge. We observe that new cell layers preferentially form at the position of +1/2 defects, whereas holes preferentially open at −1/2 defects. To explain these findings, we model the bacterial colony as an extensile active nematic fluid with anisotropic friction. In agreement with our experimental measurements, this model predicts an influx of cells towards the +1/2 defects and an outflux of cells from the −1/2 defects. Our results suggest that cell motility and mechanical cell–cell interactions are sufficient to promote the formation of cell layers at topological defects, thereby seeding fruiting bodies in colonies of M. xanthus.

土壤细菌Xanthus xanthus生活在密集的群体中，这些群体会响应环境变化形成动态的三维模式，例如饥饿期间呈液滴状的子实体¹。这些多细胞结构的发展始于对细胞层的顺序形成，这一过程鲜为人知²。在这里，使用共聚焦三维成像，我们发现了能动的杆状M. xanthus液晶盒紧密堆积并排列在每一层中，形成活性向列液晶。除了在点缺陷上携带半整数拓扑电荷以外，整个种群中的单元对齐几乎都是完美的。我们观察到，新的细胞层优先在+1/2缺陷的位置形成，而空穴优先在-1/2缺陷的位置开口。为了解释这些发现，我们将细菌菌落建模为具有各向异性摩擦的可扩展活性向列液。与我们的实验测量结果一致，该模型预测了细胞向+1/2缺陷的涌入以及细胞从-1/2缺陷的涌出。我们的结果表明，细胞运动性和细胞间机械相互作用足以促进拓扑缺陷处细胞层的形成，从而将子实体播种在Xanthus。