What are two types of bacteria that are useful for humans?

Bacteria often get an unjustly bad reputation as being harmful; while some strains of bacteria can surely lead to infections, illness, or other unpleasantness, there are many strains that are actually quite beneficial for human health.
Lactobacillus acidophilus, frequently shortened to L. acidophilus, is a bacteria that lives in the intestines of humans and other animals. One of the most common bacteria strains, which are often referred to in supplemental form as probiotics, it is able to help keep a healthy balance of bacteria in the human gut. In addition, L. acidophilus is able to help produce several B vitamins while existing in the gut.
Another important and beneficial strain for humans is Lactobacillus plantarum, a bacteria that exists naturally in fermented vegetables, such as sauerkraut or kimchi. One thing that makes this a particularly beneficial bacteria is its ability to withstand the abrasive environment of one's stomach and intestines. Where many other bacteria strains are quickly killed by the bile and acid in one's stomach, L. plantarum is able to stay around longer while helping humans with a number of intestinal sensitivities, including many types of food intolerance.