a, Staphylococcus aureus infections are notoriously difficult to treat. It is thought that this is because the bacteria enter host cells and 'hide' in intracellular compartments that conventional antibiotics cannot reach or where they are inactive. b, Lehar et al.1 covalently linked an antibiotic derivative, called a rifalogue, to an antibody that binds to components of the S. aureus cell wall. This prodrug coats the bacterial cell surface but remains inactive until the bacteria enter the host cell. There, protease enzymes cleave the linker region, releasing the active antibiotic, which then kills the bacteria.