Dr Ricard Alert

The soil bacterium Myxococcus xanthus lives in colonies of millions of cells that migrate on surfaces. When nutrients are scarce, M. xanthus colonies develop macroscopic multicellular aggregates called fruiting bodies. To develop these three-dimensional fruiting bodies, the colony sequentially forms new cell layers on top of an original cell monolayer. However, how new cell layers form is poorly understood. We find that, in each layer, the rod-shaped M. xanthus cells are densely packed, aligned with neighboring cells, and motile. Thus, the colony forms an active nematic liquid crystal. We show that new cell layers preferentially form at topological defects, i.e. points at which the nematic alignment axis is lost. Defects with topological charge +1/2 and -1/2 lead to the formation of new layers and new holes, respectively. We explain these findings by modeling the bacterial colony as an extensile active nematic fluid with anisotropic friction. In agreement with our experimental measurements, the model predicts an influx of cells toward +1/2 defects and an outflux of cells from −1/2 defects. Our results suggest that cell motility and mechanical cell-cell interactions are sufficient to induce the formation of cell layers at topological defects, thereby seeding the formation of fruiting bodies in the bacterial colony.