Microscopic, single-celled organisms that possess a prokaryotic type of cell structure, which means their cells are noncompartmentalized, and their DNA (usually circular) can be found throughout the cytoplasm rather than within a membrane-bound nucleus. They reproduce by fission or by forming spores. Bacteria can live in a wide variety of environments, such as in soil, acidic hot springs, radioactive waste, seawater, deep in the Earth's crust, in the stratosphere, and in the bodies of other organisms.

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