Interface growth and pattern formation in bacterial colonies

Bacterial species Bacillus subtilis is known to exhibit various colony patterns, depending on the substrate softness and nutrient concentration. We have examined the self-affinity and roughness exponent α for growing interfaces of colonies which belong to regions B (Eden-like) and D (disk-like) in the morphological phase diagram of this species. We have obtained α≃0.78 and 0.50 in the regions B and D, respectively, and found that the difference of α values can be attributed to that of microscopic growth mechanisms of colony interfaces. We have also proposed a unified model which produces characteristic colony patterns observed in our experiments. It is a diffusion–reaction-type model for the population density of bacterial cells and the concentration of nutrient. The essential assumption is that there exist two types of bacterial cells; active and inactive. Our model is found to be able to globally reproduce all the colony patterns in the phase diagram.