Giant enhancement of bacterial upstream swimming in macromolecular flows

Many bacteria live in natural and clinical environments with abundant macromolecular polymers. Macromolecular fluids commonly display viscoelasticity and non-Newtonian rheological behavior; it is unclear how these complex-fluid properties affect bacterial transport in flows. Here we combine high-resolution microscopy and numerical simulations to study bacterial response to shear flows of various macromolecular fluids. In stark contrast to the case in Newtonian shear flows, we found that flagellated bacteria in macromolecular flows display a giant capacity of upstream swimming (a behavior resembling fish swimming against current) near solid surfaces: The cells can counteract flow washing at shear rates up to ~65 $s^{-1}$, one order of magnitude higher than the limit for cells swimming in Newtonian flows. The significant enhancement of upstream swimming depends on two characteristic complex-fluid properties, namely viscoelasticity and shear-thinning viscosity; meanwhile, increasing the viscosity with a Newtonian polymer can prevent upstream motion. By visualizing flagellar bundles and modeling bacterial swimming in complex fluids, we explain the phenomenon as primarily arising from the augmentation of a "weathervane effect" in macromolecular flows due to the presence of a viscoelastic lift force and a shear-thinning induced azimuthal torque promoting the alignment of bacteria against the flow direction. Our findings shed light on bacterial transport and surface colonization in macromolecular environments, and may inform the design of artificial helical microswimmers for biomedical applications in physiological conditions.