Of course you mean "When strains of existing inferior bacteria that are resistent to the particular antibiotic and already present emerge in greater numbers because the superior competition for resources has been negated by the antibiotics...".
What I mean is that when a population of bacteria that exhibits variation in its genetic makeup is exposed to a selection event like toxins in their environment most die, but some survive to reproduce. It's called evolution through natural selection. Some old guy named Derwood or Darlin, or something that, discovered it more than 150 years ago!