Enhancement of bacterial rheotaxis in non-Newtonian fluids

Positive rheotaxis, swimming against flow currents, is a common trait among microorganisms such as bacteria and sperm cells, and plays an important role in many biological processes like the spread of infections and human fertility. While much is known about bacterial rheotaxis in Newtonian fluids, our understanding of it in complex fluids (e.g., mucus) remains limited. We show experimentally and theoretically that fluid shear-thinning viscosity behavior significantly enhances the upstream migration of bacteria, enabling contamination at flow rates that are nearly an order of magnitude larger than in water and Newtonian fluids. Our study and minimal model for bacterial rheotaxis near surfaces have important consequences for the study of medical, food, or wastewater contamination, and applications in designing medical devices that could prevent medical infections.