It is not supposed to kill the good bacteria, only the bad. It is also anti-fungal and anti-viral. I use it with olive leaf extract when my kids get respiratory infections and the combination knocks the infection right out as long as I am faithful to give it to them at least 3 times a day. Awesome stuff!!!
Check out North American Herb and Spice Company one of the best sources of Oregano products.
Hope this helps you out.