"Millions and millions of single-celled [...] bacteria live inside our intestines.  [...]  The survival of many species depends on the presence of bacteria.  Certain bacteria that live in our intestines [...] (intestinal microflora) help to destroy other organisms that could be harmful.  Some microflora break food down to make it easier to digest [...]; others make important vitamins, such as [...] B [...and...] K.  [...]  Scientists now believe that some important parts of the cells of animals and plants were once bacteria that were captured by some long-ago ancestor of present-day species and then were passed on by their hosts from generation to generation.  One important type is the mitochondria, found in the cells of virtually all eukaryotes (organisms whose cells have a nucleus, [which] includes plants, animals, fungi, and protists).  Mitochondria are football-shaped structures that burn food materials to release energy for the cell's activities.  Researchers have found that mitochondria have their own DNA [...].  It's different from the cell's DNA, which is specific for the type of plant or animal; in fact, it's somewhat like the DNA of bacteria."—Thank you, Alvin Silverstein, Virginia Silverstein, & Laura Silverstein Nunn, for Symbiosis, Twenty-First Century Books, 1998, pp.10-11