Curvature reversal of the circular motion of swimming bacteria probes for slip at solid/liquid interfaces

The orientation of the circular motion of most swimming E. coli at a solid surface is shown to be (i) reversed by supplementing the medium with alginate and (ii) restored by rendering the glass surface positively charged. Given slip promotion by the polymer and suppression by its anchoring on the surface, observations are interpreted by slip at the boundary. This points out bacterial swimming as a probe of the solid/fluid boundary condition.