Topological defects drive layer formation in bacteria colonies

The starvation-induced development of macroscopic fruiting bodies in Myxococcus xanthus begins with the formation of layered cell structures. Cells in these layers are densely packed, aligned via their rod shape, and retain their motility so that the population forms an active nematic liquid crystal. We investigate the origin of layering by looking at the formation of second layers and holes from cellular monolayers. These events occur at discontinuities in the cell direction field known as topological defects. Layers form at positive defects while holes open at negative defects. By measuring cell flows, we find an influx of cells towards positive defects and an outflux away from negative defects. We find that a model of monolayers as an active, dry extensile nematic with anisotropic friction can reproduce the measured flow fields and change in cell density at defects. Overall, we conclude that the conversion from a 2D cell layer to a 3D droplet is triggered by the formation of layers of cells at topological defects.