''Entamoeba'' is a
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
Amoebozoa
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 traditional and currently no longer supported classi ...
found as internal
parasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
s or
commensal
Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...
s of animals.
In 1875,
Fedor Lösch
Fyodor Alexandrovich Lesh, alternatively spelled as Lösch (russian: Фёдор Александрович Леш) (1840–1903), was a Russian Empire medical doctor.
He is credited with identifying ''Amoeba coli'' in 1875. This species was later ...
described the first proven case of
amoebic dysentery
Amoebiasis, or amoebic dysentery, is an infection of the intestines caused by a parasitic amoeba ''Entamoeba histolytica''. Amoebiasis can be present with no, mild, or severe symptoms. Symptoms may include lethargy, loss of weight, colonic u ...
in St. Petersburg, Russia. He referred to the amoeba he observed microscopically as ''Amoeba coli''; however, it is not clear whether he was using this as a descriptive term or intended it as a formal taxonomic name. The genus ''Entamoeba'' was defined by Casagrandi and Barbagallo for the species ''
Entamoeba coli
''Entamoeba coli'' is a non-pathogenic species of ''Entamoeba'' that frequently exists as a commensal parasite in the human gastrointestinal tract. ''E. coli'' (not to be confused with the bacterium ''Escherichia coli'') is important in medicin ...
'', which is known to be a
commensal
Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...
organism. Lösch's organism was renamed ''
Entamoeba histolytica
''Entamoeba histolytica'' is an anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 million people worldwid ...
'' by
Fritz Schaudinn
Fritz Richard Schaudinn (19 September 1871 – 22 June 1906) was a German zoologist.
Born in Röseningken, East Prussia, he co-discovered, with Erich Hoffmann in 1905, the causative agent of syphilis, ''Spirochaeta pallida'' (also known as ''Tr ...
in 1903; he later died, in 1906, from a self-inflicted infection when studying this amoeba. For a time during the first half of the 20th century the entire genus ''Entamoeba'' was transferred to ''
Endamoeba
''Endamoeba'' is a genus of amoeboid
An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, prima ...
'', a genus of amoebas infecting invertebrates about which little is known. This move was reversed by the International Commission on Zoological Nomenclature in the late 1950s, and ''Entamoeba'' has stayed 'stable' ever since.
Species
Several species are found in humans and animals. ''Entamoeba histolytica'' is the pathogen responsible for invasive '
amoebiasis
Amoebiasis, or amoebic dysentery, is an infection of the intestines caused by a parasitic amoeba '' Entamoeba histolytica''. Amoebiasis can be present with no, mild, or severe symptoms. Symptoms may include lethargy, loss of weight, colonic ...
' (which includes amoebic
dysentery
Dysentery (UK pronunciation: , US: ), historically known as the bloody flux, is a type of gastroenteritis that results in bloody diarrhea. Other symptoms may include fever, abdominal pain, and a feeling of incomplete defecation. Complications ...
and
amoebic liver abscess
A amoebic liver abscess is a type of liver abscess caused by amebiasis. It is the involvement of liver tissue by trophozoites of the organism ''Entamoeba histolytica'' and of its abscess due to necrosis.
Presentation
Approximately 90% of patient ...
es). Others such as ''
Entamoeba coli
''Entamoeba coli'' is a non-pathogenic species of ''Entamoeba'' that frequently exists as a commensal parasite in the human gastrointestinal tract. ''E. coli'' (not to be confused with the bacterium ''Escherichia coli'') is important in medicin ...
'' (not to be confused with ''
Escherichia coli
''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
'') and ''
Entamoeba dispar
''Entamoeba'' is a genus of Amoebozoa found as internal parasites or commensals of animals.
In 1875, Fedor Lösch described the first proven case of amoebic dysentery in St. Petersburg, Russia. He referred to the amoeba he observed microscopi ...
'' are harmless. With the exception of ''
Entamoeba gingivalis'', which lives in the mouth, and ''E. moshkovskii'', which is frequently isolated from river and lake sediments, all ''Entamoeba'' species are found in the
intestine
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...
s of the animals they infect. ''
Entamoeba invadens'' is a species that can cause a disease similar to ''E. histolytica'' but in reptiles. In contrast to other species, ''E. invadens'' forms cysts in vitro in the absence of bacteria and is used as a model system to study this aspect of the life cycle. Many other species of ''Entamoeba'' have been described, and it is likely that many others remain to be found.
Structure
''Entamoeba'' cells are small, with a single
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
* Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
and typically a single
lobose pseudopod taking the form of a clear anterior bulge. They have a simple life cycle. The trophozoite (feeding-dividing form) is approximately 10-20 μm in diameter and feeds primarily on bacteria. It divides by simple binary fission to form two smaller daughter cells. Almost all species form cysts, the stage involved in transmission (the exception is ''
Entamoeba gingivalis''). Depending on the species, these can have one, four or eight nuclei and are variable in size; these characteristics help in species identification.
Classification
''Entamoeba'' belongs to the
Archamoebae, which like many other anaerobic eukaryotes have reduced
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
. This group also includes ''
Endolimax
''Endolimax'' is a genus of amoebozoa that are found in the intestines of various animals, including the species ''E. nana'' found in humans. Originally thought to be non-pathogenic, studies suggest it can cause intermittent or chronic diarrhea ...
'' and ''
Iodamoeba'', which also live in animal intestines and are similar in appearance to ''Entamoeba'', although this may partly be due to convergence. Also in this group are the free-living amoebo-flagellates of the genus ''Mastigamoeba'' and related genera. Certain other genera of symbiotic amoebae, such as ''Endamoeba'', might prove to be synonyms of ''Entamoeba'' but this is still unclear.
Culture
Fission
Studying ''Entamoeba invadens'',
David Biron of the
Weizmann Institute of Science
The Weizmann Institute of Science ( he, מכון ויצמן למדע ''Machon Vaitzman LeMada'') is a public research university in Rehovot, Israel, established in 1934, 14 years before the State of Israel. It differs from other Israeli unive ...
and coworkers found that about one third of the cells are unable to separate unaided and recruit a neighboring amoeba (dubbed the "midwife") to complete the fission. He writes:
:''When an amoeba divides, the two daughter cells stay attached by a tubular tether which remains intact unless mechanically severed. If called upon, the neighbouring amoeba midwife travels up to 200 μm towards the dividing amoeba, usually advancing in a straight trajectory with an average velocity of about 0.5 μm/s. The midwife then proceeds to rupture the connection, after which all three amoebae move on.''
They also reported a similar behavior in ''
Dictyostelium
''Dictyostelium'' is a genus of single- and multi-celled eukaryotic, phagotrophic bacterivores. Though they are Protista and in no way fungal, they traditionally are known as "slime molds". They are present in most terrestrial ecosystems as ...
''.
Since ''E. histolytica'' does not form cysts in the absence of bacteria, ''E. invadens'' has become used as a model for encystation studies as it will form cysts under axenic growth conditions, which simplifies analysis. After inducing encystation in ''E. invadens'', DNA replication increases initially and then slows down. On completion of encystation, predominantly tetra-nucleate cysts are formed along with some uni-, bi- and tri-nucleate cysts.
Differentiation and cell biology