  In many situations bacteria move in complex environments, as for example in
soils, oceans or the human gut-track microbiome. In these natural environments,
carrier fluids such as mucus or reproductive fluids show complex structure
associated with non-Newtonian rheology. Many fundamental questions concerning
the the ability to navigate in such environments remain unsolved due to the
inherent complexity of the natural surroundings. Recently, the interaction of
swimming bacteria with nematic liquid crystals has attracted lot of attention.
In these structured fluids, the kinetics of bacterial motion is constrained by
the orientational molecular order of the liquid crystal (or director field) and
novel spatio-temporal patterns arise from this orientational constraint, as
well as from the interactions with topological defects. A question unaddressed
so far is how bacteria are able to change swimming direction in such an
environment. In this work, we study the swimming mechanism 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.
