How do antibiotics create drug-resistant strains?

Question

I've heard for years that low-level use of antibiotics causes the spread of drug-resistant strains of bacteria, but the explanations always fall short. I understand mutations and natural selection, and that a bacterium can randomly mutate into one that is more resistant to the antibiotic than the wild-type. I understand that the antibiotics kill off the less resistant strains and leave the more resistant ones, but those more resistant strains would be there even without the antibiotic use, right?Do the more resistant bacteria compete somehow with the less resistant ones and the antibiotics kill off the competition?(Edited: previously used the terms "strong" and "weak" which apparently have unintended connotations, and was unclear about the role of mutation).

Answer 1

Great question, and extremely relevant in our current practice.

In short, antibiotic resistance costs energy and resources that could be spent toward multiplying. There's a limited amount of resources available, and those bacteria with resistance are going to get out-competed by the one's without. BUT when the selective pressure, an antibiotic, is applied, ONLY those bacteria with resistance are left to consume the available resources.