A phase transition between collective and individual dynamics in suspension of swimming bacteria

We present the experimental study of the onset of collective behavior in suspension of swimming bacteria \textit{Bacillus Subtilis} in a liquid film. The system behavior is analyzed as the average swimming speed is varied in a wide range while the concentration of bacteria is constant. The average swimming speed was controlled by adjusting the concentration of dissolved oxygen in a fluid with bacteria. We obtained phase diagrams for the transition of the system between the state of collective swimming and the disordered state at different conditions.