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Article: bacteria
- Article from:
- The Columbia Encyclopedia, Sixth Edition
CopyrightThe Columbia Encyclopedia, Sixth Edition. Copyright 2008 Columbia University Press. (Hide copyright information)
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bacteria [pl. of bacterium], microscopic unicellular prokaryotic organisms characterized by the lack of a membrane-bound nucleus and membrane-bound organelles. Once considered a part of the plant kingdom, bacteria were eventually placed in a separate kingdom,
Monera
. Bacteria fall into one of two groups,
Archaebacteria
(ancient forms thought to have evolved separately from other bacteria) and Eubacteria. A recently proposed system classifies the Archaebacteria, or archaea, and the Eubacteria as major groupings (sometimes called domains) above the kingdom level.
Bacteria were the only form of life on earth for 2 billion years. They were first observed by Antony van
Leeuwenhoek
in ...