  The behavior of flagellated bacteria swimming in non-Newtonian media remains
an area with contradictory and conflicting results. We report on the behavior
of wild-type and smooth-swimming E. coli in Newtonian, shear thinning and
viscoelastic media, measuring their trajectories and swimming speed using a
three dimensional real-time tracking microscope. We conclude that the speed
enhancement in Methocel solution at higher concentration is due to
shear-thinning and an analytical model is used to support our experimental
result. We argue that shear-induced normal stresses reduce the wobbling
behavior during cell swimming but do not significantly affect swimming speed.
However, the normal stresses play an important role in decreasing the flagellar
bundling time which changes the swimming speed distribution. A dimensionless
number, the "Strangulation number" (Str) is proposed and used to characterize
this effect.
