Since bacteria are everywhere and antibiotics only work very locally to kill bacteria, your plan would not work either way. The point is: no matter how many you kill in one small place (e.g. inside one human body, one hospital, or one town) many billions more bacteria are still around in other bodies or even on the outside of the very body in which you killed everything, and also on the surfaces of most objects and everything around you. Frankly it's simply impractical to kill all bacteria and anyway you never want to do this, since, by far, bacteria do much more good than harm in the general ecology of this planet.
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