Bacteria are tiny organisms that are larger than viruses but can be seen only with the aid of a microscope. Although small, they are large in number and variety. Bacteria flourish in every possible habitat on earth--in the air, soil, and water, as well as on and within the bodies of living things. Scientists have even discovered subsurface populations of bacteria that exist as deep as 2.8 km below the Earth's surface and in locations with temperatures that reach 75 degrees C. People often associate bacteria with disease, but bacteria also perform useful functions. They are part of chemical cycles during which they release essential elements such as carbon and nitrogen for recycling. They decompose dead and decaying organic matter, help animals digest food and produce chemicals such as ethyl alcohol, butyl alcohol, acetic acid, and acetone. They aid in the production of food products such as cheese, butter, sauerkraut...