  Biased locomotion is a common feature of microorganisms, but little is known
about its impact on self-organisation. Inspired by recent experiments showing a
transition to large-scale flows, we study theoretically the dynamics of
magnetotactic bacteria confined to a drop. We reveal two symmetry-breaking
mechanisms (one local chiral and one global achiral) leading to
self-organisation into global vortices and a net torque exerted on the drop.
The collective behaviour is ultimately controlled by the swimmers' microscopic
chirality and, strikingly, the system can exhibit oscillations and memory-like
features.
