Fractal morphology facilitates Bacillus subtilis biofilm growth.

In the late stage of Bacillus subtilis biofilm growth, to adapt the extremely nutrient-lacking environment, the biofilm edge grows into a complex branching structure, which allows the biofilm to expand outward at a faster speed, comparing to the expansion speed of the biofilm edge without branching structure. The fractal analysis shows that the fractal dimension (Fd) decreases along the radius in the biofilm branching structure, as shown in Figs. 1d and 3a. The variation of Fd along the radius is not monotonic, which is because of the texture evolution induced by the bacterial clusters' movement. By using the wide field stereomicroscope and image analysis, we find that the ridges in the mature branching structure are composed of inactive substances, and most of the bacterial clusters move through the valleys. Further analysis shows that bacterial clusters move to the area with the high Succolarity (Suc) value.