'Chain and running' induced by mechanical interactions among cells of different phenotypes in the Bacillus subtilis biofilm.

In either a living system or a non-living system, the interaction among its constituent cells or particles is a fundamental aspect at all scales. For example, during the Bacillus subtilis biofilm formation, cells differentiate into multiple phenotypes to adapt to the environments; few hours after the initial inoculation, we find the phenotype of matrix-producing cells form "chain" structure surrounding the phenotype of the "running" motile cells. We use "chain" to characterize the structure of matrix-producing cells, and "running" to characterize the proliferation and growth of motile cells. Due to a large number of cells in the biofilm, it is impossible to construct a traditional kinetic model to describe the causal link between the single-cell movement and the colony behavior. Here, we obtain cell state information and cell group shape information through experiments; after the image analysis, we get the key interaction rules between cells, and then, we simulate the comparable movement of two cell types and the resulting colony geometry using the multi-agent model. Our work makes a better understanding of the relationship between the macroscopic shape of colonies and microscopic mechanical interactions among cells in the early stage of biofilm growth.