  Run-and-tumble motility is widely used by swimming microorganisms including
numerous prokaryotic eukaryotic organisms. Here, we experimentally investigate
the run-and-tumble dynamics of the bacterium E. coli in polymeric solutions. We
find that even small amounts of polymer in solution can drastically change E.
coli dynamics: cells tumble less and their velocity increases, leading to an
enhancement in cell translational diffusion and a sudden decline in rotational
diffusion. We show that suppression of tumbling is due to fluid viscosity while
the enhancement in swimming speed is mainly due to fluid elasticity.
Visualization of single fluorescently labeled DNA polymers reveals that the
flow generated by individual E. coli is sufficiently strong to stretch polymer
molecules and induce elastic stresses in the fluid, which in turn can act on
the cell in such a way to enhance its transport. Our results show that the
transport and spread of chemotactic cells can be independently modified and
controlled by the fluid material properties.
