When symbiotic gut flora populations are disrupted (e.g., by antibiotics), one becomes more vulnerable to pathogens. The rapid evolution of antibiotic resistance places an enormous selective pressure on the advantageous alleles of resistance passed down to future generations.
When gut microbial communities are disrupted (e.g., by antibiotics), one becomes more susceptible to pathogens. The rapid development of antibiotic resistance exerts a strong selective pressure on the advantageous genes responsible for resistance, which are passed down to future generations.