Three-Dimensional Characterization of Surface Wave Behavior in Dense Colonies of M. xanthus

One of the characteristic collective phenomena displayed by the social bacterium Myxococcus xanthus is its ability to self-organize into wave-like structures termed ripples. Ripples occur in both developmental and predatory phases of its life cycle, where a colony of M. xanthus is densely packed and comprised of multiple cell layers. In this work, we characterize the properties of rippling in three-dimensions. Using a surface profilometer, we demonstrate that these waves span 6 to 20 cell layers in height with a characteristic wavelength that is dependent on the stiffness of a hydrogel substrate. We find coexistence of traveling waves that move away from the colony and standing waves within denser regions of the colony. Furthermore, we use spinning-disk confocal microscopy and sparse cell labeling to understand individual cell motility and ordering within these structures. We find that the colonies are able to maintain nematic order across cell layers and produce flows with speeds that exceed those of lone cells.