Do you know how sulfa drugs (one of the first antibiotics) work? Well apparently certain cavorting wee beasties (also known as bacteria) need a vitamin that is similar in chemical construction to the sulfa drugs in order to survive. When sulfa is applied, the wee beasties think it is Thanksgiving and gorge themselves on the pseudo-food. As it turns out, bad luck for them, the sulfa compounds do not provide anything for the reproductive cycle and the bacteria end up childless and condemned to a lonely life in the bacterial equivalent of the old folks home and the grim reaper (in this case white blood cells), takes care of the rest. Therefore, no offspring: no infection.