  The rheological response under simple shear of an active suspension of
Escherichia coli is determined in a large range of shear rates and
concentrations. The effective viscosity and the time scales characterizing the
bacterial organization under shear are obtained. In the dilute regime, we bring
evidences for a low shear Newtonian plateau characterized by a shear viscosity
decreasing with concentration. In the semi-dilute regime, for particularly
active bacteria, the suspension display a "super-fluid" like transition where
the viscous resistance to shear vanishes, thus showing that macroscopically,
the activity of pusher swimmers organized by shear, is able to fully overcome
the dissipative effects due to viscous loss.
