Bacteria that are extremely helpful are bacteria of decay. Without these microorganisms, dead organic matter would never re-enter the ecosystem and enrich the soil as humus. This in turn would affect all of the operating food webs on Earth. Also, the planet would be covered in garbage and the cycles of carbon and nitrogen would be interrupted as well. Decomposers are in integral part of the biosphere. Some of the "good bacteria" in our intestines help to keep the numbers of fungi like yeast in check. Without them, an overgrowth of yeast could result in an infection. Some bacteria enable us to make foods like yogurt and cheese, via the process of fermentation. Nitrogen fixing bacteria are important because they change atmospheric nitrogen into nitrates which is a form of nitrogen plants can absorb and use to manufacture plant proteins. These can be consumed in food webs by animals which in turn can be used to make animal proteins. Bacteria get a bad reputation sometimes, but Kingdom Eubacteria have members that we just can't live without. Bacterial DNA can be genetically engineered to produce endless supplies of human growth hormone, insulin and other drugs relatively quickly to improve the human condition.
There are many types of helpful bacteria. The ones you have probably heard a lot about in the media lately are called probiotic bacteria. Some types of probiotics are Lactobacillus, Bifidobacterium and Streptococcus thermophilus. These bacteria aid in digestion and other body functions. They can be found in some dairy products. Probiotics are thought to be so helpful they can even be found as an additive in certain baby formulas.
Another group of bacteria which aid in digestion are classified as gut flora. These bacteria are found in the intestines and colons of humans. While we can live without them, they are very helpful in aiding our digestive processes.