Credit: generated using PDB file 4K19 (Allred, B. E.; Correnti, C.; Clifton, M. C.; Strong, R. K.; Raymond, K. N. ACS Chemical Biology 2013, 8, 1882-1887
A hypothetical Trojan horse drug might consist of a siderophore (blue) linked to an antibiotic, in this case the broad-spectrum antibiotic ciprofloxacin (green). Many bacteria have evolved resistance to ciprofloxacin in recent years, leaving it less effective than it once was. Attaching it to a siderophore might make the antibiotic useful again because bacteria pump siderophores inside their cell membranes where they can do maximum damage.