Wrinkle Formation

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Recent studies showed, experimentally, that localized cell death triggers the formation of wrinkles in Bacillus subtilis biofilms [1]. These studies suggest that the main effect of cell death is to break adhesive interactions between cells and the supporting agarose substratum, changing the spatial distribution of mechanical stresses in the interior of colony. The formation of wrinkles result in a reduction of the effective mechanical stress inside the biofilm. We used Biocellion to developed a simple model of the process of wrinkle formation triggered by localized cell death, focusing on the mechanical interactions inside the colony.

We setup an initial configuration in which cells are placed randomly in a regular box, see Figure. Cells are modeled by spheres of radius 4.125 micrometers, which remain constant during the simulation. The mechanical interactions are modeled by setting up elastic bonds between cells and between cells and the agarose substratum. Moreover we used periodic boundary conditions for cells. Cell death is simulated by removing the 5 central columns from the two lowest layers of cells in the central part of the colony, after 100 simulation time steps. After 10000 simulation steps a wrinkle is formed on the top of the area of cell death as shown in Figure 3 in which cells are colored according to cell height (X coordinate). The cells at the top of the wrinkle are colored in dark red.

Wrinkle formation
Although this is not a comprehensive model of wrinkle formation in biofilms, this example shows the potential of the proposed framework in linking intracellular interactions and mechanical interactions at cellular level to study colony morphology.
  1. Asally, M., Kittisopikul, M., et al. (2012). Localized cell death focuses mechanical forces during 3D patterning in a biofilm. PNAS, 109, no. 46, 18891-18896.