  Single flagellated bacteria are ubiquitous in nature. They exhibit various
swimming modes using their flagella to explore complex surroundings such as
soil and porous polymer networks. Some single-flagellated bacteria swim with
two distinct modes, one with its flagellum extended away from its body and
another with its flagellum wrapped around it. The wrapped mode has been
observed when bacteria swim under tight confinements or in highly viscous
polymeric melts. In this study we investigate the hydrodynamics of these two
modes inside a circular pipe. We find that the wrapped mode is slower than the
extended mode in bulk but more efficient under strong confinement due to a
hydrodynamic increase of its flagellum translation-rotation coupling and an
Archimedes' screw-like configuration that helps to move the fluid along the
pipe.
