Do bacteria go with the flow in bacterial turbulence?

A dense suspension of swimming bacteria displays mesmerizing jets and vortices reminiscent of inertial turbulent flow. How an individual bacterium navigates this “bacterial turbulence” generated by its neighbors remains unknown. Here, we image the motion of individual fluorescently-tagged E. coli tracers in bacterial turbulence at different bacterial densities. By comparing the motions of actively swimming cells and immobile cell bodies, we quantify the relative contributions of self-propulsion and background advection to the motion of an individual bacterium in bacterial turbulence. At densities sufficient to induce bacterial turbulence, weak advective flows are generated. At these densities, the swimming of active bacteria contributes to their motion resulting in them being faster than their immobile counterparts. However, at higher densities, the turbulent advection gets stronger, and active and immobile cells are transported at the same speed. Thus, the density of the bacterial suspension controls the coupling between the motion of an individual bacterium and the advective flows of bacterial turbulence. Finally, our findings highlight crucial differences between the transport properties of externally driven inertial turbulence and self-propulsion driven bacterial turbulence.