Antibiotics with the ability to turn themselves off might help address the problem of antibiotic accumulation in the environment, which is contributing to bacterial resistance to the drugs. In a step toward that goal, Ben L. Feringa and coworkers at the University of Groningen, in the Netherlands, show they can use light to control the activity of a modified quinolone antibiotic (Nat. Chem. 2013, DOI: 10.1038/nchem.1750). The researchers attached an azobenzene group to the quinolone, which is inactive when the azobenzene is in its trans conformation. When irradiated with ultraviolet light, the azobenzene isomerizes to the cis form, which activates the quinolone. The azobenzene group can be converted back to the trans form with visible light. The researchers show they can turn the antibiotic on and off at least three times and have it remain effective. Light isn’t necessary to turn the antibiotic off, however. After a few hours, the antibiotic turns itself off by converting thermally back to the trans form. Such self-deactivation could prevent buildup of unmetabolized active antibiotics in the environment.