Bacteria are microbes, typically 0.5 – 5.0 micrometers in length, which can be found in huge numbers throughout every part of the environment, from at least 2 miles underground to 7 miles above the surface. Bacteria colonize areas uninhabitable to most other forms of life, including Yellowstone hot springs and the McMurdo Dry Valleys of Antarctica.

Bacteria can also be found on your body, in huge numbers. It is estimated there are about ten bacteria for every cell in the human body, for a total of approximately 1 quadrillion (10) individuals, distributed among 500 to 100,000 species. The vast majority of bacteria live in the large intestine, although they live throughout the body. The body has both good bacteria, like Lactobacillus, which converts milk protein to lactic acid in the gut, and bad bacteria, like those responsible for tooth decay and stomach ulcers.

Bacteria are found in fertile soil in huge amounts: around 100 million to 3 billion per gram of soil, sometimes with up to a million species in that single gram. Although a typical soil sample is dominated by only a few species of bacteria, there are a large number present but in low abundance. These bacteria help other organisms in breaking down organic detritus, creating rich soil. In particular, bacteria are the only form of life capable of converting atmospheric oxygen (N₂ into fixed (solid) nitrogen, in the form of nitrates which are then used by plants as nutrients.

If there is organic material present somewhere, or even, in some cases, inorganic reduced compounds, then bacteria, in cooperation with other microorganisms, will likely find some way to digest it. There are bacteria that consume bits of tire on the side of the road, dissolve Alzheimer plaques in corpses, even consume TNT or sulfate minerals. The use of bacteria or other microbes to break down toxic chemicals is called bioremediation.