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Dense suspensions of swimming bacteria are known to exhibit collectivebehaviour arising from the interplay of steric and hydrodynamic interactions.Unconfined suspensions exhibit transient, recurring vortices and jets, whereasthose confinedin circular domains may exhibit order in the form of a spiralvortex. Here we show that confinement into a long and narrow macroscopic`racetrack' geometry stabilises bacterial motion to form a steadyunidirectional circulation. This motion is reproduced in simulations ofdiscrete swimmers that reveal the crucial role that bacteria-driven fluid flowsplay in the dynamics. In particular, cells close to the channel wall producestrong flows which advect cells in the bulk against their swimming direction.We examine in detail the transition from a disordered state to persistentdirected motion as a function of the channel width,and show that the width atthe crossover point is comparable to the typical correlation length of swirlsseen in the unbounded system. Our results shed light on the mechanisms drivingthe collective behaviour of bacteria and other active matter systems, andstress the importance of the ubiquitous boundaries found in natural habitats.

Directed collective motion of bacteria under channel confinement