Entamoeba is a genus of Amoebozoa found as internal parasites or commensals of animals. Several species are found in humans. Entamoeba histolytica is the pathogen responsible for amoebiasis (which includes amoebic dysentery and amoebic liver abscesses), while others such as Entamoeba 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 intestines of the animals they infect.
Entamoeba cells are small, with a single nucleus 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.
Entamoeba belongs to the Archamoebae, which are unusual in lacking mitochondria. This group also includes Endolimax, 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.
Studying Entamoeba invadens, David Biron of the Weizmann Institute of Science 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:
They also reported a similar behavior in Dictyostelium.