Bacteria are a large domain
Domain (biology)
In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...

 of prokaryotic microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

s. Typically a few micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

s in length, bacteria have a wide range of shapes, ranging from sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

s to rods and spirals. Bacteria are present in most habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

s on Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, growing in soil, acidic hot springs
Hot spring
A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are geothermal hot springs in many locations all over the crust of the earth.-Definitions:...

, radioactive waste
Radioactive waste
Radioactive wastes are wastes that contain radioactive material. Radioactive wastes are usually by-products of nuclear power generation and other applications of nuclear fission or nuclear technology, such as research and medicine...

, water, and deep in the Earth's crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

, as well as in organic matter and the live bodies of plants and animals, providing outstanding examples of mutualism in the digestive tracts of humans, termites and cockroaches.