Flow-induced bending of flagella controls bacterial surface behavior

Bacteria engage in surface-specific behaviors that are assumed to be driven by biological signaling. However, surface behaviors could be controlled by mechanical reorientation of bacterial appendages. Here, we use microfluidics and flagellar labeling to discover how shear force bends flagella to control surface behavior of the human pathogen Pseudomonas aeruginosa. By imaging flagellar rotation and using mutants with paralyzed flagella, we establish that flagellar rotation promotes surface departure in host-relevant shear regimes. Our single-cell experiments reveal two distinct subpopulations in flow: cells with their flagellum positioned upstream and cells with their flagellum positioned downstream. Shear force bends upstream flagella around the cell and blocks rotation. In contrast, downstream flagella can continue to rotate after surface arrival. Cells with downstream flagella depart the surface more frequently than cells with upstream flagella, indicating how flow direction can determine bacterial cell fate on surfaces. Together, our results demonstrate how the geometric relationship between flow and cell appendages can generate subpopulations and control surface behaviors.