Active surface waves drive rippling in Myxococcus xanthus colonies

During periods of predation or starvation, populations of the gliding bacterium Myxococcus xan-thus self-organize into striking wave-like structures termed ripples. This phenomenon was thought to arise from wave collisions triggering synchronized reversals of cell motility. However, using three-dimensional microscopy, we find no evidence for such synchronization during rippling. Instead, we show that ripples are surface waves with a period of [~] 20 min, wavelength of [~] 100 {micro}m and an amplitude of 6 to 20 cell widths at the top of a thick film of cells, akin to surface waves seen in fluids. We propose a physical model of rippling as surface waves of an active nematic liquid crystal. Two key predictions of this model are verified experimentally: the rippling wavelength increases with the surface tension at the film-air interface, and it decreases with substrate stiffness, which regulates the availability of water coating the bacterial film. These findings reveal the physical basis of rippling and highlight the role of active surface waves in shaping collective biological behavior.