Stuff that kills bacteria has to also not kill people.

I saw a TV programme where scientists had a big tray of nutrient gel. They divided the tray into sections. The fist section was normal nutrient gel. The next contained a normal dose of antibiotic. The next section was 10x, the next section was 100x and the final section was 1000x a regular dose. This was far more than humans could tolerate. They stopped at 1000x because that was the limits of solubility - they couldn't disolve more antibiotic into the gel.

Scientists then dropped a bit of e.coli on the normal gel and set their time lapse cameras.

In just a few weeks the bacteria had evolved to live on all sections of the gel.

(This was a BBC 'Horizon' programme called "defeatng the superbugs")