A unique group of microorganisms classified as bacteria (Archaeobacteria) but genetically and metabolically different from all other known bacteria. They appear to be living fossils, the survivors of an ancient group of organisms that bridged the gap in evolution between bacteria and the eukaryotes (multicellular organisms). The name Archaea comes from the Greek archaios meaning ancient.

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Ar·chaea (ahr-keґə) [Gr. archaios ancient, from archē beginning, from archein to begin] in the three-domain system of classfication, one of the two large divisions into which prokaryotes are grouped, genetically distinct from bacteria and sharing some molecular features with the eukaryotes. Organisms are diverse in shape and size, may or may not have a cell wall, and occur both as unicellular forms and as filaments or aggregations. They are subclassfied into methanogenic, sulfate-reducing, halophilic, and thermophilic groups. Members are predominantly terrestrial or aquatic organisms, living mainly in extreme environments, such as submarine hydrothermal vents and subterranean salt deposits; some occur as symbionts in the digestive tracts of animals. None are human pathogens. Cf. Bacteria.

Medical dictionary. 2011.

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