Be the flow or go with it: Dynamics of individual bacteria in bacterial turbulence

Swimming bacteria suspended in fluids at high densities display mesmerizing self-organized flows known as “bacterial turbulence.” While driving the turbulent flow, individual bacteria within bacterial turbulence display complex dynamics, which remain largely unexplored. Here, we image the motion of fluorescently-tagged E. coli cells in bacterial turbulence at different bacterial densities and compare the dynamics of actively swimming bacteria and immobile cells. At high densities, strong advective flows transport active and immobile bacteria at the same speed. However, for weaker turbulent flows at lower densities, the self-propulsion of an active bacterium competes with the background advection. This competition leads to a diversity of bacterial dynamics, ranging from being purely advected by the flow to actively swimming faster than the flow. By simultaneously imaging bacterial turbulence using bright-field and fluorescence microscopy, we quantify the coupling between the motion of an active bacterium and the background turbulent flow and identify the unusual interplay between active swimming and local advection. Taken together, our study reveals the transport of individual bacteria in dense suspensions and sheds light on the microscopic origin of bacterial turbulence.