FIG 8.

Schematic images illustrating possible mechanisms of wrinkling and the spatial organization of B. subtilis and P. agglomerans within the biofilm. (Top) Inhomogeneous growth occurs when the upper layer of the material grows faster than the interior, leading to wrinkling during growth. Buckling occurs in response to compressive stresses on materials that have already formed. Wrinkles occur only within unstable buckling regimes, when the compressive stresses are above a modulus-dependent threshold; this threshold, along with its wavelengths and amplitude, depends on both the geometry and properties of the thin sheet and its substrate (71, 72). In the stable buckling regime, the material is too stiff for the applied stresses and no buckling occurs. (Bottom) The colony morphology of each coculture is shown with the B. subtilis strain in blue (forming an elastic shell) and the P. agglomerans strain in green (forming a central “droplet” of more viscous material).