https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1007218 What this says is that in the normal human environment of US civilization convergent evolution is driving common human-associated bacteria, even those who are as yet only infrequent pathogens, i.e. not being targeted with antibiotics, to evolve mechanisms of antibiotic resistance which have no apparent vulnerability associated with them. They do not decrease fitness in any known way. They are not self clearing, in other words. They are permanent residents. If they ever become virulent, they will be antibiotic resistant from day one. If they ever exchange genetics with a pathogen, the pathogen can gain resistance immediately and without evolutionary cost.