It is thought that bacteria were among the earliest forms of life that appeared on Earth billions of years ago.
Scientists think that they helped shape and change the young planet's environment, eventually creating atmospheric oxygen that enabled other, more complex life forms to develop. Many believe that more complex cells developed as once free-living bacteria took up residence in other cells.
Bacteria organisms are made up of just one cell. They are capable of multiplying by themselves, as they have the power to divide. Their shapes vary, and doctors use these characteristics to separate them into groups.
Bacteria exists everywhere, inside and on our bodies. Most of them are completely harmless and some of them are very useful. But some bacteria can cause diseases, either because they end up in the wrong place in the body, or simply because they are designed to invade us.
Bacteria can live on or in about every material and environment on Earth from soil to water to air, and from your house to arctic ice to volcanic vents.
Each square centimeter of your skin averages about 100,000 bacteria. A single teaspoon of topsoil contains more than a billion (1,000,000,000) bacteria. And they can only be seen with the help of a microscope that magnifies them one thousand times.
They're an amazingly complex and fascinating group of creatures. Bacteria have been found that can live in temperatures above the boiling point and in cold that would freeze your blood.
They "eat" everything from sugar and starch to sunlight, sulfur and iron. There's even a species of bacteria called Deinococcus radiodurans that can withstand blasts of radiation 1,000 times greater than what would kill a human being.