Depletion-Mediated Pattern Formation in a Growing Bacterial Colony

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Secretion of extracellular polymeric substances into growth medium of bacteria is the hallmark of forming biofilm-like structures. The morphological property of such systems might depend upon the physical interactions of cells with extracellular polymeric substances(EPS). We have studied self-organization of nonmotile rod-shaped bacterial cells growing on solid substrate in presence of self-producing EPS, secreted into the growth medium in expanding colony. In our individual-based simulation model of bacterial cells and EPS, all the particles interact mechanically via repulsive forces by pushing each other away as bacterial cells grow and divide consuming diusing nutrient and produce EPS. We show that mechanical interactions control the collective behavior of the system, particularly, we show that the presence of nonadsorbing EPS leads spontaneous aggregation of bacterial cells by depletion attraction and generates phase separated patterns in a nonequilibrium growing colony. This generic mechanism powered by entropic forces could explain one of the possible ways to spontaneous aggregated structure formation and spatial heterogeneity in a biofilm.