To begin, there is no solid evidence that colloidal silver is an efficient antibiotic against either "good" or "bad" bacteria, if that is what your query implies. Despite promises made by the market, there are very few human trials because of the known dangers of using colloidal silver compounds topically or eating them.
Second, "genuine" antibiotics do not differentiate between good and bad bacteria because of their broad spectrum of activity. Antibiotic patients typically experience GI distress because the drugs kill off beneficial bacteria in the digestive tract, necessitating subsequent repopulation.