Fractal Growth and Morphological Change by a Cell Population

当少量细胞接种到营养丰富的琼脂平板(底物)表面并适当培养时，细菌菌落就会生长。我们发现，当营养物质和琼脂(控制底物柔软度)的浓度变化时，菌落模式发生了巨大的变化。因此，我们已经能够利用枯草芽孢杆菌定量地建立菌落模式的形态相图。也就是说，我们观察到了诸如自相似DLA、致密但界面粗糙的(自仿射)类伊甸园、同心环、密集分枝的DBM样以及各向同性和均匀的圆盘图案等特征图案。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活跃地移动。这种活跃的细胞运动似乎引发了一些形态上的变化。许多杆状细菌包括大肠杆菌和枯草芽孢杆菌在营养物质扩散受限的条件下，在营养不良和琼脂平板相当硬的情况下，表现出自相似的DLA模式。因此，我们可以得出结论，在这些环境条件下，DLA机制对于菌落生长是相当普遍的。对于我们所观察到的最简单的各向同性和均匀的盘状菌落，我们详细地定量测量了菌落内单个细胞的界面生长速度、种群密度和扩散系数。从而阐明了这些菌落的生长过程可以用费雪方程来描述。然而，我们还没有证实类伊甸园群体的自我亲和力，也没有阐明其他群体模式的生长机制。

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.