Is bacterial resistance really due to mutation?

I hope Crash doesn't mind me adding one important bit he left out, likely because he's been over this ground so much: bacteria have only a single set of genes, not two sets like we sexually reproducing critters. And the have little to no non-coding ("junk") DNA like we eukaryotes have. So there's no way that they can "hide" traits like antibiotic resistance in their genomes - there are no hiding places.

I'm sure that this is precisely what was done in the early days of such experiments - this, and the fact that only a tiny fraction of the poisoned bugs typically survive. Nowadays, they can actually identify the specific DNA change, the protein change it causes, and the monkey wrench that that protein change throws into the works of the antibiotic.I can imagine, too, that the survivors on a Petri dish might be well away from the spot where the great-grandma bacterium was put. That alone would strongly suggest that the resistant strain wasn't there at the point/time of colony start-up.