Active nematics and topological defects in colonies of gliding bacteria

The developmental cycle of Myxococcus xanthus involves the coordination of many hundreds of thousands of cells aggregating to form mounds known as fruiting bodies. This aggregation process begins with the sequential formation of cell layers on top of each other. We study this layer formation process by observing the formation of holes and second layers within a base monolayer of M xanthus cells. We find that these new holes and layers form at topological defects where the cell orientation field is undefined, and is driven by the bacteria acting as an active nematic liquid crystal through cell motility and substrate friction. We also track all of the individual cells within the monolayer to study the connection between individual cell motion and mean field activity within the system.