Shear prevents detachment of target cells bound to E. coli via the bacterial adhesin, FimH

Bacteria must bind to host cells in the presence of fluid flow. While the drag force associated with fluid flow will generally act to separate noncovalent bonds, we have previously reported that shear actually enhances the binding of E. coli to model target cells such as red blood cells. Here we describe the behavior of red blood cells attached to a carpet of E. coli under shear. We show that red blood cells move along or detach from the surface-bound bacteria under low shear stress, while they move much more slowly and do not detach under moderate shear stress. Finally, at higher shear stress, they move rapidly again, but still do not detach. This behavior is reversible, so that switching from high to low shear allows cells to detach and switching from low to high shear prevents the adherent cells from detaching. These observations suggest the hypothesis that FimH-receptor bonds have two states, a low and a high affinity, and that force favors a high-affinity state.