Fractal Growth and Morphological Change by a Cell Population

将少量细胞接种在营养丰富的琼脂平板（基质）表面并适当孵育时，细菌菌落生长。我们发现，当营养物和琼脂（控制基质柔软度）的浓度变化时，菌落形态会发生急剧变化。因此，我们已经能够建立定量的形态相图的菌落图案使用枯草芽孢杆菌。也就是说，我们观察到的特征模式，如自相似DLA，紧凑，但界面粗糙（自仿射）伊甸园样，同心环，密集分支的DBM样，各向同性和均匀的磁盘模式。我们还观察到，当琼脂平板变得更软时，单个细菌细胞开始能够在菌落内积极移动。这种活跃的细胞运动似乎引发了一些形态学变化。许多杆状细菌，包括大肠杆菌和枯草芽孢杆菌已被发现表现出自相似的，DLA模式由于扩散限制条件下的营养，当营养是穷人和琼脂板相当硬。因此，我们可以得出结论，DLA机制是相当普遍的菌落生长在这些环境条件下。对于我们所观察到的最简单的各向同性和均匀的圆盘集落，我们详细地定量测量了集落内单个细胞的界面生长速度、群体密度和扩散系数。因此，我们阐明了这些菌落的生长可以用Fisher方程来描述。然而，我们还没有证实伊甸园样菌落的自亲和性，也没有阐明其他菌落模式的生长机制。

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.