I think you are missing my point. If there is an antibiotic peptide that's only expressed in the brain, then there is no benefit for bacteria to evolve resistan

I think you are missing my point. If there is an antibiotic peptide that's only expressed in the brain, then there is no benefit for bacteria to evolve resistance to it. It would make us too sick. Afflicted people would stay home which would reduce spread. Normally diseases evolve the opposite way: to be less and less harmful. This is known as Smith's "law of declining virulence". So they may even evolve vulnerability. If you take that some peptide and and put it inside your whole body, then evolving resistance becomes a high priority. Bacteria who don't do it stop spreading. And once they have evolved resistance there is no going back. You can't send a security update with new brain specific anti-biotics.