Quick science lesson: there are two types of bacteria, known as gram-positive and gram-negative, that have different cell wall structures. This has presented issues for those who develop antibiotics, as the two types can mean that medicines which target one are ineffective against the other. What do you do when two kinds of walls won’t let invaders in? Build a double trojan horse, of course, of course. Clever war strategists/scientists attached antibiotics to the molecules that bacteria send out to gather iron. Once those molecules find iron and head home to their bacterium, the cell walls willingly accept the molecule, antibiotic included. But sometimes bacteria can still destroy antibiotics with enzymes, so the “double” part of the analogy kicks in—the act of destroying the first antibiotic releases the second. That’s a pretty hardcore Catch-22.