Surprising hydrodynamics of bacteria near surfaces

Recent experiments have shown that a sulfide-oxidizing bacterium named T.majus can transition from swimming in circles to a surface-bound state where it stops swimming while remaining free to move laterally along the surface. In this bound state, the cell rotates perpendicular to the surface with its flagella pointing away from it. Using simulations and theory, we demonstrate the existence of a fluid-structure interaction instability that causes cells with relatively short flagella to become surface bound. In both the bound and dynamic states, we find that bacterium gets attracted to the wall consistent with far-field force dipole assumption. However, on simply changing the shape of the cell body from a sphere T.majus to a prolate spheroid, relevant for cells such as E.coli, we find that the cell swims in circular trajectories but at a certain height above the surface.