Signal Percolation In a Population of Bacteria

Bacterial biofilms are communities in which bacteria live in a self-produced matrix and engage in remarkable emergent behaviors. A fascinating mechanism for long-distance coordination among biofilm cells is the propagation of electrical signals within the community. These signals have a population-level benefit: they halt growth of exterior cells and provide greater nutrient access to the stressed interior. We find that signaling is heterogeneous at the single-cell level. Some cells propagate the signal (“firing cells”) and others do not. In order to understand how this signal reliably propagates over hundreds-of-cells distance despite this heterogeneity, we developed a model combining percolation theory with excitable dynamics. Our model predicts that signal transmission becomes possible when firing cells are organized near a critical phase transition between a disconnected and a fully connected conduit of signaling cells, called percolation. We confirm that the spatial distribution of firing cells is organized near the predicted phase transition by measuring signaling at the single-cell level within wild-type and mutant biofilms. Our findings suggest that near this critical point, the population-level benefit of signal transmission outweighs the single-cell-level cost. The bacterial community thus appears to be organized according to a theoretically predicted spatial heterogeneity that promotes efficient signal transmission.