A time lapse movie shows an electrochemical signal propagating through a Bacillus subtilis biofilm. All cells are shown in black with intensity from a membrane potential indicator dye overlaid in color. The cells colored orange form a system-spanning cluster of spatially connected signaling cells. This is called percolation. The membrane potential of all other cells is indicated in blue. Percolation is a way for the biofilm to efficiently transmit the signal over long distances without having a majority of cells pay the individual signaling cost.