I learned this in science class:
Bacteria help us digest food and do a lot more important jobs for us. If they die out, we will probably die out as well. (That second sentence I inferred.) Cows and other grass eaters need bacteria in their stomachs, too. Otherwise, they won't be able to digest grass, and because it's the main thing they eat, they would starve. We should probably try not to make bacteria go extinct.