Medium-assisted tumbling controls bacteria exploration in a complex fluid

In nature, many fluids that harbor bacterial populations or protect against microbial contamination exhibit non-Newtonian rheology. To study, in such complex environments, the spatial exploration of <i>E. coli</i> bacteria, a model multi-flagellated microorganism, we design a motility medium with tunable macroscopic rheology. By increasing the solid charge in soft carbomer grains, we transition from a Newtonian viscous suspension to a yield-stress fluid. Using a 3D Lagrangian tracking device, we collected many individual bacterial tracks and characterized changes in motility properties such as swimming speed, persistence times and diffusivity for both a wild-type and a smooth runner mutant, up to the formation of a motility barrier at higher carbomer concentrations. We show that the presence of local mechanical disorder and resistance to penetration essentially override the biologically driven run-and-tumble navigation process. This "medium-assisted" exploration scenario is characterized by directional switching and stop-and-go kinematics and is closely related to the flexibility of the flagellar bundle.