Kingdom Monera includes the bacteria and blue-green algae. These are the simplest of all living organisms. They differ from members of the four other kingdoms because they are single-celled organisms with little organization to their cell structure. Monerans are cells with no membrane-bound structure protecting their genetic material. They reproduce by simply dividing into two separate cells.
Blue-green algae and some bacteria make their own food from sunlight, like green plants do. Some bacteria use nitrogen, sulphur, hydrogen, or iron to create their food. However most bacteria break down and absorb compounds made by other organisms. 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. In 1 g of fertile agricultural soil, there may be 2.5 billion bacteria, up to 400,000 fungi, 50,000 algae, and 30,000 protozoans.
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. Bacteria normally found in humans include Staphylococcus epidermis on the skin, Staphylococcus aureus in the nostrils, and Escherichia coli--one of the most common bacteria in the intestine.
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. In one individual Escherichia coli there are about 5,000 genes. One in 200 bacteria is likely to have a mutant characteristic on one of these genes. The ability of bacteria to quickly adapt to changing environments is the main reason for their success as an organism.