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AMPs kill bacteria by disrupting their cellular membrane. These peptides, which are positively charged, bind to the negatively charged microbes by electrostatic attraction. Bacterial cells are negatively charged because lipids compose a large part of their membrane. Once the AMPs bind to this outer wall, they insert themselves into the membrane, damaging the microbe in the process.

Miller's team found a receptor in the membrane of Salmonella that strongly attracts binding AMPs. Once triggered, the receptor, named PhoQ, turns on a large set of bacterial genes. These genes cause modifications to the membrane surface of the bacteria, eliminating the negative charge normally exhibited to the outside world. The result is that AMPs are no longer attracted to the microbe.

Salmonella have both an inner and outer membrane, which buys them some time before AMPs cause irreversible damage. This double-sided wall defines what biologists call the "gram negative" class of bacteria, and it is characteristic of many disease-causing forms of microbes. "Gram negative bacteria can have their outer membrane damaged and still recover" says Miller. "Once the inner membrane is severely damaged then you're likely to lose cellular integrity." The goal of the bacteria is to resist AMP binding before too much damage has occurred.