Enhanced upstream swimming of bacteria in complex fluids: part II, viscoelasticity

Surface-thriving bacteria are commonly found in environments that involve both shear flow and the presence of biological macromolecules. Airway mucus, for instance, contains high concentrations of polymeric mucins and exhibits non-Newtonian rheology. Having previously studied the effect of shear thinning, we started to focus on investigating the rheotaxis of E. Coli in a variety of viscoelastic fluids. Our experimental findings demonstrated efficient upstream propulsion and enhanced upstream swimming in these viscoelastic fluids under shear flow. To shed light on the underlying mechanisms, we developed a theoretical model showing how fluid elasticity enhances bacteria's ability to swim upstream. We propose that, in DNA suspensions, both the shear-thinning effect and fluid elasticity work synergistically to enhance the weathervane effect, thereby facilitating bacteria to swim upstream. This study of bacterial motion in polymeric flow uncovers fundamental aspects of bacterial transport and infection dynamics in viscoelastic environments such as the respiratory flows.