  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 starvation. The development of
these multicellular structures begins with the sequential formation of cell
layers in a process that is poorly understood. 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 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 promote
the formation of cell layers at topological defects, thereby seeding fruiting
bodies in colonies of M. xanthus.
