Pleomorphism is a concept that two or more structural forms can occur during a life cycle

Biological life cycle

A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...

, especially of certain plants, including many marine plants such as Mastocarpus stellatus

Mastocarpus stellatus

Mastocarpus stellatus, also called Clúimhín Cait , carragheen, or false Irish moss, is a species of red algae closely related to Irish Moss, or Chondrus crispus. It is collected in Ireland and Scotland, together with Chondrus crispus as Irish Moss, dried, and sold for cooking and as the basis for a...

. It can also apply at the species level.

Cellular differentiation

Cellular differentiation

In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...

 in multicellular organisms, where the young organism changes form when developing, is not considered pleomorphism, since only the new cells have a different form, while the original cell has been divided in two.

Extreme Pleomorphism is the claim that life forms can interchange among themselves between bacteria to viruses and fungi.

Bacteria

In the first decades of the 20th century, the term was used to refer to the supposed ability of bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 to change shape dramatically or to exist in a number of extreme morphological (changing) forms. This claim sparked a controversy among the microbiologists

Microbiology

Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...

 and split them into two schools: the monomorphists, who opposed the claim, and the pleomorphists (such as Antoine Béchamp

Antoine Béchamp

Pierre Jacques Antoine Béchamp was a French biologist. He studied silkworm parasites, and was the first to synthesise Atoxyl.-Biography:...

, Günther Enderlein

Günther Enderlein

Günther Enderlein was a German zoologist, entomologist and later a manufacturer of pharmaceutical products. Enderlein got some international renown because of his insect research but in Germany he became famous because of his concept of the pleomorphism of microorganisms and his hypotheses about...

 or even Albert Calmette

Albert Calmette

Léon Charles Albert Calmette ForMemRS was a French physician, bacteriologist and immunologist, and an important officer of the Pasteur Institute. He discovered the Bacillus Calmette-Guérin, an attenuated form of Mycobacterium used in the BCG vaccine against tuberculosis...

http://books.google.fr/books?id=QdEsl0f88U4C&pg=PA128&dq=bovo+vaccination&hl=fr&ei=KhUhTPTfMZyTsQbazbmjDg&sa=X&oi=book_result&ct=result&resnum=9&ved=0CE0Q6AEwCDgK#v=onepage&q=bovo%20vaccination&f=false).

Monomorphic theory, supported by Louis Pasteur

Louis Pasteur

Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

, Rudolf Virchow

Rudolf Virchow

Rudolph Carl Virchow was a German doctor, anthropologist, pathologist, prehistorian, biologist and politician, known for his advancement of public health...

, Ferdinand Cohn

Ferdinand Cohn

Ferdinand Julius Cohn was a German biologist.Cohn was born in Breslau in the Prussian Province of Silesia. At the age of 10 he suffered hearing impairment. He received a degree in botany in 1847 at the age of nineteen at the University of Berlin. He was a teacher and researcher at University of...

, and Robert Koch

Robert Koch

Heinrich Hermann Robert Koch was a German physician. He became famous for isolating Bacillus anthracis , the Tuberculosis bacillus and the Vibrio cholerae and for his development of Koch's postulates....

, emerged to become the dominant paradigm

Paradigm

The word paradigm has been used in science to describe distinct concepts. It comes from Greek "παράδειγμα" , "pattern, example, sample" from the verb "παραδείκνυμι" , "exhibit, represent, expose" and that from "παρά" , "beside, beyond" + "δείκνυμι" , "to show, to point out".The original Greek...

 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 modern-day definition of pleomorphism in the context of bacteria is now a variation of size or shape of the cell, rather than a change of shape as previously believed.

Extreme pleomorphism - where bacteria can turn into viruses and fungi has been claimed to be found in a 1970 paper by Virginia Livingston

Virginia Livingston

Virginia Livingston was an American physician and cancer researcher who advocated the unsupported theory that a specific species of bacteria she named Progenitor cryptocides was the primary cause of cancer in humans...

 However Livingston's findings have been discarded along with her treatment methods and cancer theory which has been disproved.

Viruses

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 hepatitis B virus.

Gaston Naessens

Pleomorphism was claimed to have been demonstrated by Canadian researcher Gaston Naessens on the bacterial and sub-bacterial level, including new types of life which he claimed to have discovered called Somatids. Naessens claims to have developed a new type of microscopic technique.