Fractal Growth and Morphological Change by a Cell Population

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Bacterial colonies grow when a small amount of the cells are inoculated on the surface of nutrient-rich agar plates (substrates) and incubated appropriately. We have found that colony patterns change drastically when concentrations of nutrient and agar (controlling substrate softness) are varied. We have thus been able to establish quantitatively the morphological phase diagram of colony patterns by using Bacillus subtilis. Namely, we observed characteristic patterns such as self-similar DLA, compact but interfacially rough (self-affine) Eden-like, concentric-ring, densely branched DBM-like, and isotropic and homogeneous disk patterns. We also observed that individual bacterial cells start to able to move actively inside colonies when agar plates become softer. This active cell movement seems to trigger some morphological changes. Many rod-like bacterial species including Escherichia coli and Bacillus subtilis have been found to exhibit self-similar, DLA patterns due to the diffusion-limited condition of nutrient, when nutrient is poor and agar plates are rather hard. We can, therefore, conclude that the DLA mechanism is fairly universal for colony growth under these environmental conditions. For the isotropic and homogeneous disk colonies which are the simplest patterns we have ever observed we quantitatively measured the interfacial growth speed, population density and diffusion coefficient of individual cells inside colonies in detail. We thus elucidated that the growth of these colonies can be described by the Fisher's equation. However, we have not yet confirmed the self-affinity of Eden-like colonies, nor elucidated the growth mechanisms for other colony patterns.