There are two kind bacteria if see from the uses for human, benefit and disadvantage for human. Most of us think of bacteria as "germs" or microorganism, unseen threats to our health and welfare. But for each of the few disease-producing kinds which can live in the human body, there are hundreds of others upon which we depend for our very existence. Bacteria help us digest our food. Plant will die if no bacteria to convert leaf or other organic matter to humus.
More detail the Useful of Bacteria
