Capillary attraction facilitates bacterial collective dynamics: Theory

Myxococcus xanthus is a bacterium that lives in soil which often traps and stores water, and where capillary forces can be substantial. Key to the life cycle of M. xanthus cells is the formation of collective groups: they feed on prey in swarms and aggregate upon starvation. In the latter case, small motile groups of cells coarsen into large cell monolayers, ultimately forming three-dimensional aggregates called fruiting bodies. However, the physical mechanisms that drive the early coarsening process remain unclear. Here, we developed a computational model to study the role of capillary forces in this process. Our results demonstrate that water menisci forming around M. xanthus cells mediate a strong attraction between cells. Such capillary attraction, combined with cell motility, results in a variety of phases of collective motion, including “streams” and “diffusing droplets.” In agreement with experiment, our results show that capillary attraction facilitates mergers of cell groups and hinders their splitting, hence influencing large-scale coarsening dynamics.