Flow-driven delivery boosts antibiotic effectiveness by overwhelming bacterial defenses

Physical, chemical, and biological factors collectively determine antibiotic effectiveness. However, laboratory studies often consider only one or two of these factors and fail to capture key interaction effects. Here, we use microfluidics to study the interplay of host-relevant shear flow, the antibiotic gentamicin, and the human pathogen Pseudomonas aeruginosa. We discover bacterial populations can defend against gentamicin by chemical inactivation or physical sequestration. In low flow regimes, bacterial defenses succeed by eliminating gentamicin faster than it is delivered. As flow increases, delivery overwhelms bacterial defenses, allowing gentamicin to penetrate populations and inhibit cells. Combining biophysical simulations and long-channel microfluidics, we demonstrate that antibiotic susceptibility depends on spatial context, as cells at the beginning of the channel shield cells at the end. Surprisingly, we show that populations of sensitive cells can successfully defend against gentamicin, resulting in spatial gradients that are shaped by flow and cell density. Collectively, our interdisciplinary experiments reveal how flow overwhelms bacterial defenses, providing a framework to better understand antibiotic effectiveness in dynamic environments.