Bacteria regularly evolve antibiotic resistance, but little is known about this process at the population level. Here, a continuous culture of Escherichia coli facing increasing antibiotic levels is followed. Most isolates taken from this population are less antibiotic resistant than the population as a whole. A few highly resistant mutants provide protection to the less resistant constituents, in part by producing the signalling molecule indole, which serves to turn on drug efflux pumps and oxidative-stress protective mechanisms.