Figure 5.

(a) During the early ripple phase, bacteria form a large disc-like population. Counter-rotating waves propagate along the rim of the colony as streams form to feed aggregation centers—precursors of fruiting bodies—distributed along the colony rim about a wavelength apart. The Inset shows the fruiting bodies being fed by the radial component of flow (frame from ref. 10). (b and c) Counterpropagating wave train computed from the model Eqs. 3 and 5 in contour and perspective plots. The tilt in the waves is caused by the density gradient between the proximal (Top) and peripheral (Bottom) regions of the colony, which causes a slow drift of individuals outward, bringing with it cells with retarded phases. This phase-coupling causes the waves to tilt: right-moving waves tilt right, and left-moving waves tilt left. Where the waves intersect, the population density is doubled. The intersection regions move downward to the colony periphery, where they seed the incipient fruiting body aggregates with a spacing about equal to the wavelength of the two wave trains.