Basically the RNA changes protein(s) within the bacteria making them drug resistant. Alot of the reasons are still unknown at this time. It doesn't seem to be related to the antibiotics (but that could change and it could vary as to the species of bacteria - the major problem now is the "golden" Stayh. aureus species resistance even to the "big gun" antibiotic Vancomyacin - the antibiotic of last resort.)
I think science and medicine are running scare right now on what is transpiring - it's getting out-of-hand! I know I am.
Google will bring up some good articles - but suggest you read the watered down versions, as I do, from Science and Nature. More to come I hope. But right now it's the protein within the bacteria changing and as mentioned by "BigB" (post #4033) its not considered a "real mutation" because it is not pasted on to future bacteria (?) - so its not evolution but it does work for the "bugs". Rick