Pleomorphism, in microbiology, is the ability of some bacteria to alter their shape or size in response to environmental conditions. Pleomorphism has been observed in some members of the Deinococcaceae family. The modern definition of pleomorphism in the context of bacteriology is based on variation of size or shape of the cell, rather than a change of shape as previously believed.
In the first decades of the 20th century, the term "pleomorphism" was used to refer to the idea that bacteria changed shape dramatically or existed in a number of extreme morphological forms. This claim was controversial among microbiologists of the time, and split them into two schools: the monomorphists, who opposed the claim, and the pleomorphists such as Antoine Béchamp, Ernst Almquist, Günther Enderlein and Albert Calmette.
Monomorphic theory, supported by Louis Pasteur, Rudolf Virchow, Ferdinand Cohn, and Robert Koch, emerged to become the dominant paradigm in modern medical science: it is now almost universally accepted that each bacterial cell is derived from a previously existing cell of practically the same size and shape. However it has recently been shown that certain bacteria are capable of dramatically changing shape, for example Helicobacter pylori exists as both a helix-shaped form (classified as a curved rod) and a coccoid form.
The virions of certain viruses are sometimes seen to express pleomorphism, in the sense that they can show variable appearances. However, this characteristic is in fact not a true pleomorphic characteristic, since one and the same virion doesn't change shape, although its successors might take another shape. One example is the bacterial viruses of the Plasmaviridae family.
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