An amoeba (/əˈmiːbə/; rarely spelled amœba; plural am(o)ebas or am(o)ebae /əˈmiːbi/),[1] often called amoeboid, is a type of cell or unicellular organism which has the ability to alter its shape, primarily by extending and retracting pseudopods.[2] Amoebas do not form a single taxonomic group; instead, they are found in every major lineage of eukaryotic organisms. Amoeboid cells occur not only among the protozoa, but also in fungi, algae, and animals.[3][4][5][6][7]

Microbiologists often use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement.[8][9]

In older classification systems, most amoebas were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow. However, molecular phylogenetic studies have shown that Sarcodina is not a monophyletic group whose members share common descent. Consequently, amoeboid organisms are no longer classified together in one group.[10]

The best known amoeboid protists are the "giant amoebae" Chaos carolinense and Amoeba proteus, both of which have been widely cultivated and studied in classrooms and laboratories.[11][12] Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, and the multicellular "social amoeba" or slime mould Dictyostelium discoideum.

Amoeba collage
Clockwise from top right: Amoeba proteus, Actinophrys sol, Acanthamoeba sp., Pompholyxophrys sp., Euglypha sp., neutrophil ingesting bacteria

Shape, movement and nutrition

The forms of pseudopodia, from left: polypodial and lobose; monopodial and lobose; filose; conical; reticulose; tapering actinopods; non-tapering actinopods

Amoebae move and feed by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell.[13]

The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another. Amoebozoan species, such as those in the genus Amoeba, typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section. Cercozoan amoeboids, such as Euglypha and Gromia, have slender, thread-like (filose) pseudopods. Foraminifera emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as the Radiolaria and Heliozoa, have stiff, needle-like, radiating axopodia (actinopoda) supported from within by bundles of microtubules.[3][14]

Amoeba (PSF)
Morphology of a naked lobose amoeba

Free-living amoebae may be "testate" (enclosed within a hard shell), or "naked" (also known as gymnamoebae, lacking any hard covering). The shells of testate amoebae may be composed of various substances, including calcium, silica, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms.[15]

Difflugia acuminata
Shell of the testate amoeba Difflugia acuminata.

To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell.[16] This organelle is necessary because freshwater has a lower concentration of solutes (such as salt) than the amoeba's own internal fluids (cytosol). Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst.

Marine amoebae do not usually possess a contractile vacuole because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water.[17]

Phagocytosis -- amoeba
Amoeba phagocytosis of bacteria

The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and other protists. Some are detritivores and eat dead organic material.

Amoebae typically ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, and there is no fixed place on the cell at which phagocytosis normally occurs.[18]

Some amoebae also feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane .[19]

Size range

Ammonia tepida
Foraminifera have reticulose (net-like) pseudopods, and many species are visible with naked eye

The size of amoeboid cells and species is extremely variable. The marine amoeboid Massisteria voersi is just 2.3 to 3 micrometres in diameter,[20] within the size range of many bacteria.[21] At the other extreme, the shells of deep-sea xenophyophores can attain 20 cm in diameter.[22] Most of the free-living freshwater amoebae commonly found in pond water, ditches and lakes are microscopic, but some species, such as the so-called "giant amoebae" Pelomyxa palustris and Chaos carolinense, can be large enough to see with the naked eye.

Species or cell type Size in micrometres
Massisteria voersi[20] 2.3–3
Naegleria fowleri[23] 8–15
Neutrophil (white blood cell)[24] 12–15
Acanthamoeba[25] 12–40
Entamoeba histolytica[26] 15–60
Arcella vulgaris[27] 30–152
Amoeba proteus[28] 220–760
Chaos carolinense[29] 700–2000
Pelomyxa palustris[30] up to 5000
Syringammina fragilissima[31] up to 200000

Amoebae as specialized cells and life cycle stages

Neutrophil with anthrax copy
Neutrophil (white blood cell) engulfing anthrax bacteria

Some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, and engulf them by phagocytosis.[32]

Amoeboid stages also occur in the multicellular fungus-like protists, the so-called slime moulds. Both the plasmodial slime moulds, currently classified in the class Myxogastria, and the cellular slime moulds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage. The amoeboid cells of the former combine to form a giant multinucleate organism,[33] while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating "slug" which functions as a single organism.[8]

Other organisms may also present amoeboid cells during certain life-cycle stages, e.g., the gametes of some green algae (Zygnematophyceae)[34] and pennate diatoms,[35] the spores (or dispersal phases) of some Mesomycetozoea,[36][37] and the sporoplasm stage of Myxozoa and of Ascetosporea.[38]

Amoebae as organisms

Early history and origins of Sarcodina

Der Kleine Proteus from Roesel
The first illustration of an amoeboid, from Roesel von Rosenhof's Insecten-Belustigung (1755).

The earliest record of an amoeboid organism was produced in 1755 by August Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus" ("the Little Proteus").[39] Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as Amoeba proteus.[40] The term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid.[41]

In 1822, the genus Amiba (from the Greek ἀμοιβή amoibe, meaning "change") was erected by the French naturalist Bory de Saint-Vincent.[42][43] Bory's contemporary, C. G. Ehrenberg, adopted the genus in his own classification of microscopic creatures, but changed the spelling to Amoeba.[44]

In 1841, Félix Dujardin coined the term "sarcode" (from Greek σάρξ sarx, "flesh," and εἶδος eidos, "form") for the "thick, glutinous, homogenous substance" which fills protozoan cell bodies.[45] Although the term originally referred to the protoplasm of any protozoan, it soon came to be used in a restricted sense to designate the gelatinous contents of amoeboid cells.[10] Thirty years later, the Austrian zoologist Ludwig Karl Schmarda used "sarcode" as the conceptual basis for his Division Sarcodea, a phylum-level group made up of "unstable, changeable" organisms with bodies largely composed of 'sarcode.'[46] Later workers, including the influential taxonomist Otto Bütschli, emended this group to create the class Sarcodina,[47] a taxon that remained in wide use throughout most of the 20th century.

Within the traditional Sarcodina, amoebae were generally divided into morphological categories, on the basis of the form and structure of their pseudopods. Amoebae with pseudopods supported by regular arrays of microtubules (such as the freshwater Heliozoa and marine Radiolaria) were classified as Actinopods; whereas those with unsupported pseudopods were classified as Rhizopods.[48] The Rhizopods were further subdivided into lobose, filose, and reticulose amoebae, according to the morphology of their pseudopods.

Dismantling of Sarcodina

In the final decade of the 20th century, a series of molecular phylogenetic analyses confirmed that Sarcodina was not a monophyletic group. In view of these findings, the old scheme was abandoned and the amoebae of Sarcodina were dispersed among many other high-level taxonomic groups. Today, the majority of traditional sarcodines are placed in two eukaryote supergroups: Amoebozoa and Rhizaria. The rest have been distributed among the excavates, opisthokonts, and stramenopiles. Some, like the Centrohelida, have yet to be placed in any supergroup.[10][49]


Recent classification places the various amoeboid genera in the following groups:

Supergroups Major groups and genera Morphology
  • Lobose pseudopods (Lobosa): Lobose pseudopods are blunt, and there may be one or several on a cell, which is usually divided into a layer of clear ectoplasm surrounding more granular endoplasm.
  • Filose pseudopods (Filosa): Filose pseudopods are narrow and tapering. The vast majority of filose amoebae, including all those that produce shells, are placed within the Cercozoa together with various flagellates that tend to have amoeboid forms. The naked filose amoebae also includes vampyrellids.
  • Reticulose pseudopods (Endomyxa): Reticulose pseudopods are cytoplasmic strands that branch and merge to form a net. They are found most notably among the Foraminifera, a large group of marine protists that generally produce multi-chambered shells. There are only a few sorts of naked reticulose amoebas, notably the gymnophryids, and their relationships are not certain.
  • Radiolarians are a subgroup of actinopods that are now grouped with rhizarians.
  • The heterokont chrysophyte and xanthophyte algae includes some amoeboid members, the latter being poorly studied.[51]
  • Parasite with amoeboid life cycle stages.

Some of the amoeboid groups cited (e.g., part of chrysophytes, part of xanthophytes, chlorarachniophytes) were not traditionally included in Sarcodina, being classified as algae or flagellated protozoa.

Pathogenic interactions with other organisms

Trophozoites of Entamoeba histolytica with ingested erythrocytes
Trophozoites of the pathogenic Entamoeba histolytica with ingested red blood cells

Some amoebae can infect other organisms pathogenically, causing disease:


Recent evidence indicates that several Amoebozoa lineages undergo meiosis.

Orthologs of genes employed in meiosis of sexual eukaryotes have recently been identified in the Acanthamoeba genome. These genes included Spo11, Mre11, Rad50, Rad51, Rad52, Mnd1, Dmc1, Msh and Mlh.[53] This finding suggests that the ‘’Acanthamoeba‘’ are capable of some form of meiosis and may be able to undergo sexual reproduction.

The meiosis-specific recombinase, Dmc1, is required for efficient meiotic homologous recombination, and Dmc1 is expressed in Entamoeba histolytica.[54] The purified Dmc1 from E. histolytica forms presynaptic filaments and catalyses ATP-dependent homologous DNA pairing and DNA strand exchange over at least several thousand base pairs.[54] The DNA pairing and strand exchange reactions are enhanced by the eukaryotic meiosis-specific recombination accessory factor (heterodimer) Hop2-Mnd1.[54] These processes are central to meiotic recombination, suggesting that E. histolytica undergoes meiosis.[54]

Studies of Entamoeba invadens found that, during the conversion from the tetraploid uninucleate trophozoite to the tetranucleate cyst, homologous recombination is enhanced.[55] Expression of genes with functions related to the major steps of meiotic recombination also increase during encystations.[55] These findings in E. invadens, combined with evidence from studies of E. histolytica indicate the presence of meiosis in the Entamoeba.

Dictyostelium discoideum in the supergroup Amoebozoa can undergo mating and sexual reproduction including meiosis when food is scarce.[56][57]

Since the Amoebozoa diverged early from the eukaryotic family tree, these results suggest that meiosis was present early in eukaryotic evolution. Furthermore, these findings are consistent with the proposal of Lahr et al.[58] that the majority of amoeboid lineages are anciently sexual.


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External links

Amoeba's Secret

Amoeba's Secret is an EP by Paul McCartney recorded during a secret performance at Amoeba Music in Hollywood, California, on 27 June 2007. The EP was released in November 2007 in a limited vinyl edition, and in January 2009 on CD and download formats.The EP reached 119 on the Billboard 200 albums chart, despite relatively little promotion. The low-resolution album art is intentional, as to make the EP resemble a bootleg recording. The back of the sleeve features an incomplete word search whose letters contain hidden details of the recording.

Amoeba (genus)

Amoeba is a genus of single-celled amoeboids in the family Amoebidae. The type species of the genus is Amoeba proteus, a common freshwater organism, widely studied in classrooms and laboratories.

Amoeba (operating system)

Amoeba is a distributed operating system developed by Andrew S. Tanenbaum and others at the Vrije Universiteit Amsterdam. The aim of the Amoeba project was to build a timesharing system that makes an entire network of computers appear to the user as a single machine. Development at the Vrije Universiteit was stopped: the source code of the latest version (5.3) was last modified on 30 July 1996.The Python programming language was originally developed for this platform.

Amoeba Culture

Amoeba Culture (Hangul: 아메바 컬쳐) is a South Korean independent record label established in 2006 by Dynamic Duo (Gaeko and Choiza) and Go Kyung-min. The label is home to prominent artists such as Dynamic Duo, Primary, Rhythm Power, Crush, and Ha:tfelt. The label had the third largest sales among Korean record labels in 2015.

Amoeba Music

Amoeba Music is a US independent music chain with stores in Berkeley, San Francisco, and Hollywood, Los Angeles, California. It was founded in 1990 in San Francisco's Haight-Ashbury district and remains in operation, having survived the decline of CD sales in the 2000s.

Amoeba Wars

Amoeba Wars is a 1981 board game published by Avalon Hill.

Amoeba defense

The amoeba defense is a defensive strategy in the game of basketball.

Amoeba proteus

Amoeba proteus (core of Amoeba), alternatively Chaos diffluens, is an amoeba closely related to the giant amoebae and a species commonly bought at science supply stores.

This small protozoan uses tentacular protuberances called pseudopodia to move and phagocytose smaller unicellular organisms,(which may be greater in size than of amoeba), which are enveloped inside the cell's cytoplasm in a food vacuole, where they are slowly broken down by enzymes. Amoeba proteus is very well known for its extending pseudopodia. It occupies freshwater environments and feeds on other protozoans, algae, rotifers, and even other smaller amoebae. Due to phytochromes, A. proteus may appear in a variety of colors (often yellow, green and purple) under a microscope.

Latin words are hard to translate, but Amoeba Proteus can be translated as nucleus of Amoeba or Amoeba itself.

A. proteus possesses a nucleus containing granular chromatin, and is therefore a eukaryote. Its membrane is a phospholipid bilayer similar to other Eukaryote organisms.


Amoebiasis, also known amoebic dysentery, is an infection caused by any of the amobae of the Entamoeba group. Symptoms are most common during infection by Entamoeba histolytica. Amoebiasis can be present with no, mild, or severe symptoms. Symptoms may include abdominal pain, diarrhea, or bloody diarrhea. Complications can include inflammation and ulceration of the colon with tissue death or perforation, which may result in peritonitis. People affected may develop anemia due to loss of blood.Cysts of Entamoeba can survive for up to a month in soil or for up to 45 minutes under fingernails. Invasion of the intestinal lining results in bloody diarrhea. If the parasite reaches the bloodstream it can spread through the body, most frequently ending up in the liver where it can cause amoebic liver abscesses. Liver abscesses can occur without previous diarrhea. Diagnosis is typical by stool examination using a microscope, but may not reliably exclude infection or separate between specific types. An increased white blood cell count may be present in severe cases. The most accurate test is finding specific antibodies in the blood, but it may remain positive following treatment. Bacterial colitis can result in similar symptoms.Prevention of amoebiasis is by improved sanitation, including separating food and water from faeces. There is no vaccine. There are two treatment options depending on the location of the infection. Amoebiasis in tissues is treated with either metronidazole, tinidazole, nitazoxanide, dehydroemetine or chloroquine, while luminal infection is treated with diloxanide furoate or iodoquinoline. Effective treatment against all stages of the disease may require a combination of medications. Infections without symptoms do not require treatment but infected individuals can spread the parasite to others and treatment can be considered. Treatment of other Entamoeba infections apart from E. histolytica is not needed.Amoebiasis is present all over the world, though most cases occur in the developing world. About 480 million people are infected with amoebiasis and this results in the death of between 40,000–110,000 people a year. Most infections are now believed due to E. dispar. E. dispar is more common in certain areas and symptomatic cases may be less common than previously reported. The first case of amoebiasis was documented in 1875 and in 1891 the disease was described in detail, resulting in the terms amoebic dysentery and amoebic liver abscess. Further evidence from the Philippines in 1913 found that upon swallowing cysts of E. histolytica volunteers developed the disease.


Amoebozoa is a major taxonomic group containing about 2,400 described species of amoeboid protists, often possessing blunt, fingerlike, lobose pseudopods and tubular mitochondrial cristae. In most classification schemes, Amoebozoa is ranked as a phylum within either the kingdom Protista or the kingdom Protozoa. In the classification favored by the International Society of Protistologists, it is retained as an unranked "supergroup" within Eukaryota. Molecular genetic analysis supports Amoebozoa as a monophyletic clade. Most phylogenetic trees identify it as the sister group to Opisthokonta, another major clade which contains both fungi and animals as well as some 300 species of unicellular protists. Amoebozoa and Opisthokonta are sometimes grouped together in a high-level taxon, variously named Unikonta, Amorphea or Opimoda.Amoebozoa includes many of the best-known amoeboid organisms, such as Chaos, Entamoeba, Pelomyxa and the genus Amoeba itself. Species of Amoebozoa may be either shelled (testate), or naked, and cells may possess flagella. Free-living species are common in both salt and freshwater as well as soil, moss and leaf litter. Some live as parasites or symbiotes of other organisms, and some are known to cause disease in humans and other organisms.

While the majority of amoebozoan species are unicellular, the group also includes several varieties of slime molds, which have a macroscopic, multicellular stage of life during which individual amoeboid cells aggregate to produce spores.

Amoebozoa vary greatly in size. Some are only 10–20 μm in diameter, while others are among the largest protozoa. The well-known species Amoeba proteus, which may reach 800 μm in length, is often studied in schools and laboratories as a representative cell or model organism, partly because of its convenient size. Multinucleate amoebae like Chaos and Pelomyxa (the so-called "giant amoebae") may be several millimetres in length, and some multicellular amoebozoa, such as the "dog vomit" slime mold Fuligo septica, can cover an area of several square meters.


Arcellinid testate amoebae or Arcellinida, Arcellacean or lobose testate amoebae are single-celled protists partially enclosed in a simple test (shell).

Arcellinid testate amoebae are commonly found in soils, leaf litter, peat bogs and near/in fresh water. They use their pseudopodia (false feet), a temporary cell extension, for moving and taking in food. Like most amoebae, they are generally believed to reproduce asexually via binary fission. However a recent review suggests that sexual recombination may be the rule rather than the exception in amoeboid protists in general, including the Arcellinid testate amoebae.

Chaos (genus)

Chaos is a genus of amoebae, in the family Amoebidae. The largest and best-known species, the so-called "giant amoeba" (Chaos carolinense), can reach lengths of 5 mm, although most specimens fall between 1 and 3 mm.Members of the genus closely resemble Amoeba and share the same general morphology, producing numerous cylindrical pseudopods, each of which is rounded at the tip. However, while Amoeba have a single nucleus, Chaos can have as many as a thousand. Because of this attribute, C. carolinensis was once placed in the genus Pelomyxa along with the other giant multinucleate amoeba, Pelomyxa palustris. Recently, molecular phylogenetic studies of this species have confirmed the view of some earlier researchers that it is more closely related to Amoeba than to Pelomyxa. The species is now placed in the independent genus Chaos, a sister group to Amoeba.


The Hartmannellidae are a family of amoebozoa, usually found in soils. When active they tend to be roughly cylindrical in shape, with a single leading pseudopod and no subpseudopodia. This form somewhat resembles a slug and as such they are also called limax amoebae. Trees based on rRNA show the Hartmannellidae as usually defined are paraphyletic to the Amoebidae, which may adopt similar forms.

List of The Powerpuff Girls secondary characters

This is a list of secondary characters that appeared on the American animated television series The Powerpuff Girls, which aired on Cartoon Network. All of the characters on this list appear more than once in the series, and contribute to the episode plotlines.

Naegleria fowleri

Naegleria fowleri, colloquially known as the "brain-eating amoeba", is a species of the genus Naegleria, belonging to the phylum Percolozoa. It is a free-living, bacteria-eating amoeba that can be pathogenic, causing a fulminant (sudden and severe) brain infection called naegleriasis, also known as primary amoebic meningoencephalitis (PAM). This microorganism is typically found in bodies of warm freshwater, such as ponds, lakes, rivers, and hot springs. It is also found in the soil near warm-water discharges of industrial plants, and in unchlorinated or minimally-chlorinated swimming pools. It can be seen in either an amoeboid or temporary flagellate stage.

Nelder–Mead method

The Nelder–Mead method (also downhill simplex method, amoeba method, or polytope method) is a commonly applied numerical method used to find the minimum or maximum of an objective function in a multidimensional space. It is a direct search method (based on function comparison) and is often applied to nonlinear optimization problems for which derivatives may not be known. However, the Nelder–Mead technique is a heuristic search method that can converge to non-stationary points on problems that can be solved by alternative methods.The Nelder–Mead technique was proposed by John Nelder and Roger Mead in 1965, as a development of the method of Spendley et al.


Pandoravirus is a genus of giant virus, first discovered in 2013. It is the second largest in physical size of any known viral genus. Pandoraviruses have double stranded DNA genomes, with the largest genome size of any known viral genus.

Space Amoeba

Space Amoeba (ゲゾラ・ガニメ・カメーバ 決戦! 南海の大怪獣, Gezora, Ganime, Kamēba: Kessen! Nankai no Daikaijū, lit. "Gezora, Ganimes, and Kamoebas: Decisive Battle! Giant Monsters of the South Seas"), is a 1970 Japanese science fiction kaiju film directed by Ishirō Honda, and featuring special effects by Sadamasa Arikawa. The film tells the story of extraterrestrial amoeba-like aliens that hijack a probe and, after crash landing on a small South Pacific atoll, create gigantic monsters from native lifeforms (a kisslip cuttlefish, stone crab and mata mata) with plans of conquering the Earth.

The film was released theatrically in the United States in 1971 by American International Pictures as Yog-Monster From Space.


The Tubulinea are a major grouping of Amoebozoa, including most of the larger and more familiar amoebae like

Amoeba, Arcella, and Difflugia.

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