  During development, organisms acquire three-dimensional shapes with important
physiological consequences. While the basic mechanisms underlying morphogenesis
are known in eukaryotes, it is often difficult to manipulate them in vivo. To
circumvent this issue, here we present a study of developing Vibrio cholerae
biofilms grown on agar substrates in which the spatiotemporal morphological
patterns were altered by varying the agar concentration. Expanding biofilms are
initially flat, but later experience a mechanical instability and become
wrinkled. Whereas the peripheral region develops ordered radial stripes, the
central region acquires a zigzag herringbone-like wrinkle pattern. Depending on
the agar concentration, the wrinkles initially appear either in the peripheral
region and propagate inward (low agar concentration) or in the central region
and propagate outward (high agar concentration). To understand these
experimental observations, we developed a model that considers diffusion of
nutrients and their uptake by bacteria, bacterial growth/biofilm matrix
production, mechanical deformation of both the biofilm and the agar, and the
friction between them. Our model demonstrates that depletion of nutrients
beneath the central region of the biofilm results in radially-dependent growth
profiles, which in turn, produce anisotropic stresses that dictate the
morphology of wrinkles. Furthermore, we predict that increasing surface
friction (agar concentration) reduces stress anisotropy and shifts the location
of the maximum compressive stress, where the wrinkling instability first
occurs, toward the center of the biofilm, in agreement with our experimental
observations. Our results are broadly applicable to bacterial biofilms with
similar morphologies and also provide insight into how other bacterial biofilms
form distinct wrinkle patterns.
