Bacteria are probably the most common form of life on earth. They are also the simplest and smallest living things. They are micro-organism, which mean they can be seen only under microscope.
Some bacteria cause disease, but most do not. There are at least 2000 species of bacteria. Most of them are harmless or helpful to other forms of life including people. Others live on fallend leaves or animal wastes, in water, milk and most food, and no dust and soil. Some bacteria can cause hydrogen gas, ammonia and iron compounds as their food. A few bacteria feed on gases and acids that are poisonous to us.
Most bacteria are killed by heat, yet they are a few kinds that live in