Frustrated run and tumble of swimming E-coli bacteria in nematic liquid crystals*

In many situations bacteria move in complex environments associated with non-Newtonian rheology. In liquid crystals the kinetics of bacterial motion is constrained by the orientational molecular order of the fluid and new dynamics arise from this orientational constraint. In this work, we study the swimming reorientation of a single bacterium, E. coli, constrained to move along the director field of a lyotropic chromonic liquid crystal (LCLC) that is confined to a planar cell. In such an environment, the spontaneous run and tumble motion of the bacterium gets frustrated: the elasticity of the liquid crystal prevents flagella from unbundling. Interestingly, in order to change direction, bacteria execute a reversal motion along the director field, driven by the relocation of a single flagellum to the other side of the bacterial body, coined as a frustrated tumble. We present a detailed experimental characterization of this phenomenon, exploiting exceptional spatial and temporal resolution of bacteria and flagella dynamics during swimming, obtained using a two color Lagrangian tracking technique. We suggest a possible mechanism behind the frustrated run and tumble motion, accounting for these observations.