Colonies of bacteria grown on agar plates can display morphological patterns of surprising varieties depending on culture conditions. For instance, B. subtilis shows a dense-branching morphology on a nutrient-poor semi-solid agar medium, while some chemotactic strains of E. coli form a spot pattern on a pseudo-hexagonal lattice. Based on such experimental data, diffusion-reaction models that incorporate cell movements and interactions between bacterial cells and chemical substances have been constructed, and used to explain how the observed patterns are generated.