  From intracellular protein trafficking to large scale motion of animal
groups, the physical concepts driving the self-organization of living systems
are still largely unraveled. Selforganization of active entities, leading to
novel phases and emergent macroscopic properties, recently shed new lights on
these complex dynamical processes. Here we show that, under the application of
a constant magnetic field, motile magnetotactic bacteria confined in
water-in-oil droplets self-assemble into a rotary motor exerting a torque on
the external oil phase. A collective motion in the form of a large-scale
vortex, reversable by inverting the field direction, builds-up in the droplet
with a vorticity perpendicular to the magnetic field. We study this collective
organization at different concentrations, magnetic fields and droplets radii
and reveal the formation of two torque-generating areas close to the droplet
interface. We characterize quantitatively the mechanical energy extractable
from this new biological and self-assembled motor.
