Bacterial resistance to single antibiotics often leads to cross-resistance to other antibacterial agents. By evolving independent Escherichia coli populations in the presence of one of several different antibiotics in vitro, Lázár et al. now show that the evolution of resistance to one antibiotic can also be accompanied by hypersensitivity to other antibiotics. Populations that were adapted to aminoglycosides (which target the ribosome) showed increased sensitivity to many other antibiotic classes, including those that target the cell wall and DNA synthesis. Whole-genome sequencing revealed that mutations that disturb the proton motive force (PMF) across the inner membrane were particularly common in the resistant strains, and biochemical assays indicated that these mutations diminish the activity of the PMF-dependent AcrAB multidrug efflux pump, resulting in hypersensitivity to several other antibiotics.