This is actually akin to a antibiotic, it's a protein, an enzyme that a bacterophage virus produces to attack bacteria. See a bit more here: http://en.wikipedia.org/wiki/Lysin Also see all the antibiotics that are peptides in the Wikipedia list: http://en.wikipedia.org/wiki/List_of_antibiotics A peptide is basically a small/short protein, by convention less than 50 amino acids.

One issue which is noted in the Wikipedia article is that it's subject to attack by the immune system. Maybe that won't be a big problem, or it can be gimmicked, but it's still a big protein, would require topical or IV instead of oral dosing, and it's not going to be able to get to a variety of places that much smaller mold, indeed fungal produced antibiotics can get into.

I'm curious as to why bacteria didn't evolve to resist Lysin?

Well, perhaps some have (haven't looked at this closely), however, since in nature the various types of it are only used by specific bacteriophages, i.e. as I understand it they are or tend to be species specific, using a particular one against other species of bacteria should be fruitful.

Then again, interspecies transfer of resistance through e.g. plasmids happens.