Viscosity of suspension of swimming bacteria

Measurements of the shear viscosity in suspensions of swimming \textit{Bacillus subtilis} in free standing liquid films have revealed that the viscosity can decrease by up to a factor of seven compared to the viscosity of the same liquid without bacteria or with non-motile bacteria. The reduction in viscosity is observed in two complimentary experiments: one studying the decay of a large vortex induced by a moving probe and another measuring the viscous torque on a rotating magnetic particle immersed in the film. The viscosity depends on the concentration and swimming speed of the bacteria. The viscosity reduction is attributed to the effect of self- propulsion of swimming bacteria.