The rippling of myxobacteria: a mathematical model

Under starvation conditions myxobacteria aggregate into large, flat colonies that coalesce into many fruiting bodies. During this aggregation phase the colony develops a cooperative phenomenon called 'rippling'. This appears as a pattern of waves on the surface of the colony that propagate continuously for long periods. The wave crests are 10-20 body lengths apart, and appear to pass through one another with no interference. The waves can persist with no net mass transport, analogous to water waves. Development of the ripple phase requires both the social (S) and asocial (A) motility systems, as well as the C-signalling system of intercellular communication. This signalling system operates by direct cell contact, without any diffusible chemotactic ligands. I will present a model for the ripple phenomenon based on the observation that individual cells appear to posses an internal cycle that manifests itself in individuals as periodic reversals in direction with no net progress in either direction. Understanding the mechanism of ripple formation reveals how intracellular dynamics, intercellular communication, and cell motility can coordinate to produce collective behavior. This pattern of waves is quite different from that observed in other social bacteria, especially Dictyostelium discoidum, that depend on diffusible morphogens.