Entamoeba is a
genusIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
of
AmoebozoaThe Amoebozoa are a major group of amoeboid protozoa, including the majority that move by means ofinternal cytoplasmic flow. Their pseudopodia are characteristically blunt and finger-like,...
found as internal
parasiteParasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the host....
s or
commensalIn ecology, Commensalism is a class of relationship between two organisms where one organism benefits but the other is unaffected. There are two other types of association: mutualism and parasitism .Commensalism derives from the English word commensal, meaning "sharing of food" in human social...
s of animals.
In 1875, Fedor Lösch described the first proven case of
amoebic dysenteryAmoebic dysentery is a type of dysentery caused primarily by the amoeba Entamoeba histolytica.Amoebic dysentery is transmitted through contaminated food and water. Amoebae spread by forming infective cysts which can be found in stools and spread if whoever touches them does not sanitize their hands...
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 coliEntamoeba coli is a non-pathogenic species of Entamoeba that frequently exists as a commensal parasite in the human gastrointestinal tract. Clinically, E...
, which is known to be a
commensalIn ecology, Commensalism is a class of relationship between two organisms where one organism benefits but the other is unaffected. There are two other types of association: mutualism and parasitism .Commensalism derives from the English word commensal, meaning "sharing of food" in human social...
organism.
Entamoeba is a
genusIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
of
AmoebozoaThe Amoebozoa are a major group of amoeboid protozoa, including the majority that move by means ofinternal cytoplasmic flow. Their pseudopodia are characteristically blunt and finger-like,...
found as internal
parasiteParasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the host....
s or
commensalIn ecology, Commensalism is a class of relationship between two organisms where one organism benefits but the other is unaffected. There are two other types of association: mutualism and parasitism .Commensalism derives from the English word commensal, meaning "sharing of food" in human social...
s of animals.
In 1875, Fedor Lösch described the first proven case of
amoebic dysenteryAmoebic dysentery is a type of dysentery caused primarily by the amoeba Entamoeba histolytica.Amoebic dysentery is transmitted through contaminated food and water. Amoebae spread by forming infective cysts which can be found in stools and spread if whoever touches them does not sanitize their hands...
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 coliEntamoeba coli is a non-pathogenic species of Entamoeba that frequently exists as a commensal parasite in the human gastrointestinal tract. Clinically, E...
, which is known to be a
commensalIn ecology, Commensalism is a class of relationship between two organisms where one organism benefits but the other is unaffected. There are two other types of association: mutualism and parasitism .Commensalism derives from the English word commensal, meaning "sharing of food" in human social...
organism. Lösch's organism was renamed
Entamoeba histolytica by
Fritz SchaudinnFritz Richard Schaudinn was a German zoologistBorn in Röseningken, East Prussia, he co-discovered, with Erich Hoffmann in 1905, the causative agent of syphilis, Spirochaeta pallida...
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, 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.
Entamoeba histolyticaEntamoeba histolytica is an anaerobic parasitic protozoan, part of the genus Entamoeba. Predominantly infecting humans and other primates, E. histolytica is estimated to infect about 50 million people worldwide...
is the pathogen responsible for '
amoebiasisEntamebiasis is a term for the infection more commonly known as amoebiasis.It became the preferred term in MeSH in 1991, but the term amoebiasis is used by the World Health Organization and by those working in the field of amoebiasis research....
' (which includes amoebic
dysenteryDysentery is an inflammatory disorder of the intestine, especially of the colon, that results in severe diarrhea containing mucus and/or blood in the feces. If left untreated, dysentery can be fatal....
and amoebic liver abscesses), while others such as
Entamoeba coliEntamoeba coli is a non-pathogenic species of Entamoeba that frequently exists as a commensal parasite in the human gastrointestinal tract. Clinically, E...
(Not to be confused with Escherichia coli) and
E. dispar 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
intestineIn anatomy, the intestine is the segment of the alimentary canal extending from the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
s of the animals they infect.
Structure
Entamoeba cells are small, with a single
nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
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
E. 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
ArchamoebaeThe Archamoebae are a group of Amoebozoa distinguished by the absence of mitochondria. They include two genera, Entamoeba and Endolimax, that are internal parasites or commensals of animals. A few species are human pathogens, causing diseases such as amoebic dysentery...
, which are unusual in lacking
mitochondriaIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5–10 micrometers in diameter...
. This group also includes
EndolimaxEndolimax is a genus of amoebozoa that are found in the intestines of various animals, including the species E. nana found in humans. It causes no known disease, though it is very significant in medicine because it can provide false positives for other tests, notably the similar species Entamoeba...
, which also lives in animals and is similar in appearance to
Entamoeba, although this may partly be due to convergence. Certain other genera of symbiotic amoebae, such as
Endamoeba, might prove to be synonyms of
Entamoeba but this is still unclear.
Fission
Studying
Entamoeba invadens, David Biron of the
Weizmann Institute of ScienceThe Weizmann Institute of Science , known as Machon Weizmann is a university and research institute in Rehovot, Israel...
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
DictyosteliumThe dictyostelids are a group of cellular slime molds, or social amoebae.-Slug behavior:When food, normally bacteria, is readily available they are individual amoebae, which feed and divide normally...
.
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