Aggregation during fruiting body formation in Myxococcus xanthus is driven by reducing cell movement

When starved, Myxococcus xanthus cells assemble themselves into aggregates of about 10(5) cells that grow into complex structures called fruiting bodies, where they later sporulate. Here we present new observations on the velocities of the cells, their orientations, and reversal rates during the early stages of fruiting body formation. Most strikingly, we find that during aggregation, cell velocities slow dramatically and cells orient themselves in parallel inside the aggregates, while later cell orientations are circumferential to the periphery. The slowing of cell velocity, rather than changes in reversal frequency, can account for the accumulation of cells into aggregates. These observations are mimicked by a continuous agent-based computational model that reproduces the early stages of fruiting body formation. We also show, both experimentally and computationally, how changes in reversal frequency controlled by the Frz system mutants affect the shape of these early fruiting bodies.