Symbiosis
Symbiosis is an intimate association between two different organisms. In fact, most animals and plants live symbiotically with microorganisms. The larger organism is called the host and smaller organism or organisms the symbionts. Examples include bacterial colonization of the skin and digestive tract of animals and the roots of plants. For the microorganism, the benefits of the association can be a stable protective environment provided by the host. The bacteria may also obtain nutrients from the host. On the other hand, the symbionts can "protect" the host by making it more difficult for colonization by pathogenic bacteria. Some symbionts supply the host with nutrients that the host cannot synthesize itself nor obtain from its food.
Click on the Symbiont Hosts at left (or below) to learn more about various animal:bacterial symbioses.