Bacteria (singular: bacterium) are unicellular
microorganisms. Typically a few
micrometres in length, individual bacteria have a wide-range of shapes, ranging from
spheres to rods to spirals. Bacteria are ubiquitous in every
habitat on
Earth, growing in soil,
acidic hot springs,
radioactive waste, seawater, and deep in the
Earth's crust. There are typically 40 million bacterial
cells in a gram of soil and a million bacterial cells in a millilitre of fresh water; in all, there are approximately five nonillion (5×1030) bacteria in the world. Bacteria are vital in recycling nutrients, and many important steps in
nutrient cycles depend on bacteria, such as the
fixation of nitrogen from the
atmosphere. However, most of these bacteria have not been characterised, and only about half of the
phyla of bacteria have species that can be
cultured in the
laboratory. The study of bacteria is known as
bacteriology, a branch of
microbiology.
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