Bacteria are necessary organisms in most ecological systems. The cycling of nitrogen, which is essential for plant and animal growth, depends upon bacteria. Dead and dying material is broken down by bacteria and fungi, allowing nutrients to be recycled. Bacteria or algae form the main food source for many other organisms. Bacteria can be found from hot springs to deep-sea vents, and from deserts to polar ice.
Many bacteria live in association with plants and animals.
Bacteria that normally live in an animal may survive on dead skin cells, in digested food in the intestines, or by absorbing nutrients directly from living tissues. They benefit the host animal by aiding digestion, producing vitamins and minerals, and inhibiting the growth of harmful bacteria. Most bacteria reproduce by simply splitting in two. Under the best conditions, a population of Escherichia coli can double in number every 12.5 minutes. Bacteria can multiply so rapidly that mutations appear and allow the population to adapt to changing conditions.