Once again, the bacteria are not mutating. It is just that the stronger strains have always been resistant to drugs but previously were so tiny in numbers (and possibly could not as easily compete for food as well as other strains) that our immune system could deal with them. Now, the weaker strains that were vunerable to the antibiotics are all gone and the drug resistant strains are all that is left.
Speculation or fact?
Even in a genetically similar population, mutations occur and after many generations, the genetic profile of the bacteria has changed. The resistance to antibiotics is specifically from mutations that have been selected for by our use of antibiotics.
IOW, the weaker strains are now stronger?
Have they evolved so that our immune systems can no longer take care of them?
Can you say "they are evolving"?
E-V-O-L-U-T-I-O-N.
Their DNA is a combination of the DNA from multiple ancestor bacteria. The DNA has recombined, it has mutated, it has diverged and been selected for and it has evolved.
They sequence these genomes. It's on paper. It is irrefutable. It follows common sense. It is scientific. It is fact. It is evolution.
Explain how this happens when an experiment starts with a single bacterium. Duh.