Bacteria are microorganisms that many times get a negative reputation, however, most are harmless and some we couldn't live without. There are bacteria that produce vitamins, help with digestion and occupy niches that otherwise would be home to harmful bacteria. Probiotics are bacteria found in yogurt and are actually good for your digestive system. Bacteria of decay help with decomposition of dead organisms, thereby returning chemical compounds that would otherwise be lost, back to the environment. Some bacteria are nitrogen-fixing bacteria and can convert atmospheric nitrogen into nitrates that plants can use to synthesize plant proteins. This is an important part of the nitrogen cycle. Bacteria are necessary for the process of fermentation and products like cheese and sauerkraut are made utilizing bacteria. The link below provides additional references including books and articles regarding the importance of bacteria.