Bacteria Navigate Anisotropic Media Using A Flagellar Tug-of-Oars

Bacterial navigation of anisotropic fluids plays a vital role in biofilm formation, bacterial colonization of the mucosal linings of lungs and reproductive tract, and the organization of the gut microbiome. In these settings, hydrodynamic interactions force bacteria to swim along a preferred direction, rather than the classical run-and-tumble walk in three dimensions. Using nematic phase DSCG liquid crystal as a model system, we have observed a novel swimming mechanism employed by peritrichous bacteria to explore anisotropic fluids. Bacteria adopt a polar flagella configuration along the direction of nematic alignment that resembles a 'Tug-of-Oars' between opposing bundles. Further, they can exchange flagella between bundles and reverse directions by buckling flagella during motor reversals (tumbles). The critical compression for the Euler buckling of a flagellum is determined by the Frank energies for liquid crystal deformation. At high DSCG concentrations, this forbids buckling and rearrangement of flagella. Despite this limitation, bacteria can still alternate swimming directions by activating/deactivating bundles on opposite sides of the cell body. Our results shed light on bacterial strategies to navigate anisotropic media, and raise questions on signal transduction in flagella motors located on the cell membrane.