The effect of flow on swimming bacteria controls the initial colonization of curved surfaces

The colonization of solid surfaces by bacteria is a widespread phenomenon with major consequences on environmental processes, biotechnology and human health. While much is known about the molecular mechanisms of surface colonization, the influence of the physical environment remains poorly understood. Here we show that the magnitude and location of colonization of non-planar surfaces by motile bacteria is largely controlled by the local flow conditions. Using microfluidic experiments with Pseudomonas aeruginosa and Escherichia coli , we demonstrate that the velocity gradients created by a curved surface drive preferential attachment to specific regions of the collecting surface, namely the leeward side of cylinders and immediately downstream of apexes on corrugated surfaces, locations that are in stark contrast to where non-motile cells attach. The preferential attachment location depends on the local hydrodynamic conditions and, as revealed by a mathematical model benchmarked on the observations, on cell morphology and swimming traits, while it is independent of the physicochemical properties of the surface. The interplay between imposed flow and bacterial motility further affects the overall attachment rate, increasing it by up to two orders of magnitude compared to the non-motile case at moderate flow velocities of up to twenty times the bacterial swimming speed. These results highlight the importance of fluid flow on the magnitude and location of bacterial colonization of surfaces and provide a mechanistic model to predict colonization in flow for a wide range of applications, from infection biology to bioremediation and biofouling.