(a) Illustration of the model. (b) Snapshots of the colonies of pure E. coli and pure A. baylyi at t = 16. A colony of E. coli expanded only slightly, while a pure colony of A. baylyi expanded quickly, but remained circular. (c) Colony radius vs time for the mixed and single-species colonies. Radius is defined as . (d) Several snapshots of E. coli density during the growth of a mixed colony in simulations. (e) Colony snapshots at time t = 16 in simulations using different friction parameters. For larger friction, the colony grew slower, but still featured flower-like patterns. For smaller friction, the colony expanded more quickly, but patterns eventually disappeared. However, increasing the initial concentration of E. coli at low friction coefficients restored patterning. (f) Experimental snapshots with different agar concentrations 16 hr after inoculation: similar phenomenology observed.