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Bacteria (singular: bacterium) are unicellular microorganisms. They are typically a few micrometres long and have many shapes including spheres, rods, and spirals. The study of bacteria is bacteriology, a branch of microbiology. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste,[1] seawater, and deep in the earth's crust. Some bacteria can even survive in the extreme cold and vacuum of outer space. 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 (5×10<sup>30</sup>) bacteria in the world.[2] 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.[3]