You miss the point. I'm not referring to your technique in your experiment (sad that I even have to explain that fact). I'm referring to evidence that the resistant trait *already* existed in the population.
Your experiment merely culls an existing population.
I'm referring to evidence that the resistant trait *already* existed in the population.If your version would be true and the trait 'already existed', the bacteria would have to have (1) accuired a susceptibility after we isolated the first bacterium and tested its descendants on the antibiotic, and then (2) reverted back to its pre-existent resistant form when we started the main part of the experiment. The bacteria would, in other words, have evolved twice between two known forms.