I read that many common disease germs have evolved resistance to antibiotic drugs. Hence, our drug industry is constantly searching for newer antibiotics, to replace the older ones which have lost effectiveness. My question: as these disease bacteria evolve, do they sometimes lose their invulnerability to the older antibiotics? So can older drugs (like penicillin) regain usefulness?
Older antibotics are still used and useful. Not all bacteria have become resistant to pennicillin. It’s often ordered first, for low grade infections. If it works, the job is done, if it doesn’t, a change to a newer one usually works.
I don’t think any antibiotics have fallen into such disfavor that they are no longer used. (with the possible exception of some of the sulfas, but that’s for other reasons.)
The answer is yes. The genes that code for antibiotic resistance are just wasted DNA if the bacteria is never exposed to that antibiotic, and the bacterial populations will lose them over time.
This effect is hurt by the fact that many drugs fall into families in which resistance to one antibiotic of that family infers resistance to many others.
The same thing is true of parasite resistance to wormers. There is now a big push to stop worming animals all the time so that parasites aren’t exposed to them and the non-resistant parasites become the majority of the population.