Colonies of the bacteria E. coli and Salmonella grown under conditions of scarce food form a wide variety of spatiotemporal patterns. We model the bacterial system with autonomous nonlinear PDE's for bacterial density, nutrient (stimulant) density, and the density of a third species, a chemoattractant which is secreted by the bacteria, and towards which they are attracted. Analysis and simulation of the model reveal pattern formation in three very different regimes: (1) transient spots in liquid medium, (2) stationary rings and rings of spots in semisolid medium, (3) moving rings with trailing spots and stripes in semisolid medium.

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