Bacteria are ubiquitous, microscopic organisms that are typically only a few microns in length and consist of just a single cell. They are thought to have been among the first life forms and are classified as neither plants nor animals but as prokaryotes.
Bacteria are present in almost all habitats, including the intestines and skin of humans and other animals, soil, water, and even in acidic hot springs, hydrothermal vents, radioactive waste and deep in the earth's crust. They are vital to the existence of plants and animals in part because of their role in many stages of the nutrient cycle by recycling nutrients, including the decomposing of dead organisms and the fixation of nitrogen from the atmosphere.
Bacteria also have many important uses related to human activities, including the production of yogurt and cheese through fermentation, the production of antibiotics and other substances, sewage treatment, the breakdown of oil spills, and the recovery of gold, palladium, copper and other metals.