In microbiology, pleomorphism (from greek πλέω- more, and -μορφή form) is the ability of some micro-organisms 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. According to a 1997 journal article by Milton Wainwright, a British microbiologist, Pleomorphism of bacteria lacked wide acceptance among modern microbiologists of the time.
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.
There are also other cases of bacteria exhibiting pleomorphism. In one study that was focused on agents involved in a non-infectious disease, Pleomorphic bacteria were found to exist in the blood of healthy human subjects. 
One factor that affects the pleomorphism of some Bacteria is their nutrition. For example, the bacterium Deinococcus Radiodurans has been shown to exhibit pleomorphism in relation to differences in the nutrient contents of its environment. 
The virions of certain viruses sometimes exhibit pleomorphism, in the sense that their appearances can vary. However, this is not true pleomorphism, as individual virions are not changing shape, but being succeeded by virions with different shapes. One example is the bacterial viruses of the Plasmaviridae family.
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