Bacterial biofilms are unusual examples of active excitable matter. They grow due to cell divisions (a motile process) and can modulate their membrane potentials. Knowledge on how the clinically relevant Gram-negative bacterium, Escherichia coli, uses its membrane potential dynamics is scarce. We report for the first time, community-level synchronized membrane potential dynamics in three-dimensional E. coli biofilms. We reveal that the voltage-gated Kch-potassium channel mediates both local spiking of single E. coli cells and long-range coordinated electrical signaling in E. coli biofilms. Our data provide robust information on the membrane potential dynamics in E. coli and presents an experimental method for studying collective electrical oscillations in E. coli. The electrical phenomena are explained using Hodgkin-Huxley and 3D fire-diffuse-fire agent-based models. The electrical oscillations suggest a mechanism by which E. coli biofilms become resilient to light stress.