Specifically, there are a bunch of new antibiotic resistant bacteria evolving out there. As we move on to different antibiotics to kill those strains, and they in turn evolve to resist those antibiotics as well, is it possible that there may come a time when they’re susceptible to the original antibiotics again? Full circle here doesn’t necessarily mean they have to be genetically identical.
I’m not an expert, but it only took a few decades for antibiotic resistance to evolve. If resistance to antibiotics XYZ loses its survival value, that trait will go down. But I don’t know if it would ever truly disappear from earth.
I assume some bacteria will still retain plasmids which code for resistance for the forseeable future, so those genes can be transferred back into population if they are needed again rather than evolved from scratch.
According to Ray Kurzweil, sometime in the 2020s (which I think is optimistic) nanobots that can be injected into the body will be medically possible, and one of the possible tasks of these bots will be to kill pathogenic bacteria. I don’t know if bacteria could ever evolve a way to defeat microscopic robots which can literally tear the bacteria apart via various means. But like I said, 2020 sounds optimistic, but I wouldn’t be surprised if something like that existed in the 2040s.
Strains that are no longer exposed to the original antibiotic could quite easily lose resistance to it. Often the mutations that give resistance may not be the optimal ones to have with respect to other aspects of growth and survival. Once the alleles for resistance doesn’t confer superior survival to the bacteria, it might be out-competed by other alleles. However, as long as there is some exposure to the antibiotic, the genes will persist.