Subphylum Sarcodina. Genus Entamoeba

The amoeboid parasites, which move and feed by pseudopodia, and the genus Entamoeba within them; Entamoeba histolytica, its trophozoite and cyst, its faecal-oral transmission, the amoebiasis it causes and how it invades the intestinal wall and reaches the liver, lungs and brain; the morphology of its two forms, including the ingested erythrocytes, the four nuclei of the mature cyst, the chromatoid body and the karyosome; its life cycle and what is suspected about its sexual process; and the other species of Entamoeba found in humans, Entamoeba dispar, Entamoeba coli and Entamoeba gingivalis.

These are the amoeboid members of the phylum Sarcomastigophora, the subphylum Sarcodina. In them the cytoplasm forms temporary protrusions, pseudopodia, for locomotion and food intake, and this kind of movement is called amoeboid.

The name of this genus means "internal amoeba". Its members are either commensals or intestinal parasites. So the genus takes its name from where it lives, and its species range from harmless lodgers to one of the most dangerous intestinal parasites of humans.

• This parasite inhabits the large intestine of humans.
• It has two forms: trophozoite (active) and cyst (quiescent).
• The cyst can survive in the environment for weeks. It is the infective form.
• The infection is transmitted through the faecal-oral route by ingesting cysts shed with the faeces of a previous host (the slides write it foecal-oral).
• Excystation (transformation of cysts into trophozoites) occurs in the intestine.

Most infected people are asymptomatic carriers, but some develop a disease called amoebiasis (amoebic dysentery), which is characterised by abdominal pain, bloody diarrhoea, and lesions in the intestines.

The reason is that the parasites secrete proteases, with the help of which they enter the intestinal epithelium, where they feed on blood cells (which are often found in the amoeba cells cytoplasm), and from there, through the bloodstream, they reach other organs (liver, lungs, brain) and cause dangerous complications - abscesses, which without treatment quickly lead to death.

E. histolytica infection is estimated to kill more than 55,000 people each year.

The accompanying figure traces the route: the cysts are ingested by mouth, the parasite establishes itself in the large intestine, and from there red arrows run to the liver, the lungs and the brain, which are the three sites of the abscesses.

The proteases are the key to the whole clinical picture. They are what turns a gut commensal-like organism into an invasive one, and everything downstream, the bloody diarrhoea, the ulcers, the distant abscesses, follows from the ability to digest a way through the epithelium.

• The trophozoite is amoeboid; ingested erythrocytes distinguish it from related commensal species. This is the single most useful diagnostic feature: a commensal amoeba does not eat blood cells.
• The cyst is spherical. When mature, it has 4 nuclei. In its cytoplasm, ribosomes form an aggregation called chromatoid body.
• In both forms, chromatin is seen in two locations in the nucleus: at the periphery and in a central cluster called karyosome. (Do not study these details.)

The drawings label the red blood cells being digested inside the trophozoite, and the chromatoid body and the karyosome inside the cyst. The micrographs show a trophozoite with engulfed erythrocytes, a cyst, and a trophozoite and a cyst side by side in which the cyst wall is visible.

The contaminated food is fruit and vegetables with cysts on their surface.

The cycle in the drawing runs:

• Ingestion in contaminated food and water.
• The mature cyst reaches the intestine.
• Excystation: one trophozoite with four nuclei emerges, divides three times and each nucleus divides once, to produce eight trophozoites from each cyst.
• The trophozoites migrate to the large intestine.
• Trophozoites multiply by binary fission.
• Encystation, giving first the immature cyst and then the quadrinucleate cyst.
• Noninvasive infection: cysts exit the host in the stool.
• Invasive infection: trophozoites invade the intestinal mucosa, and then spread through the bloodstream, infecting sites such as the liver, brain, and lungs.

So the cycle has two branches from the same starting point: the parasite either stays in the lumen and is passed on as cysts, or invades, which is the branch that produces disease.

Like the metamonads, E. histolytica has genes related to recombination, and it is speculated that it undergoes meiosis during the conversion from the uninucleate trophozoite to quadrinucleate cyst; however, the sexual process has not yet been studied.

The suspicion is reasonable on the face of it: a single nucleus becoming four is the sort of event meiosis would explain, and the genes are there. But the lecture is careful to call it speculation, and so should you.

Entamoeba dispar

• Nonpathogenic species in the large intestine.
• Similar to E. histolytica but never invades the intestinal tissue to feed on blood cells.
• Trophozoite and cyst forms.

Entamoeba coli

• Nonpathogenic, in the large intestine.
• Does not move much by its pseudopodia and keeps its round shape.
• Trophozoite and cyst forms.

Entamoeba gingivalis

• Found between the gingival pockets and near the base of the teeth.
• In 95% of people with gum disease and in 50% of people with healthy gums.
• Direct transmission by kissing or by sharing eating utensils.
• Trophozoite and cyst forms.

E. gingivalis is of particular interest to dental students, both because of how common it is and because the lecture points to a video of it phagocytosing a polymorphonuclear leucocyte.

• Amoeboid protozoans move and feed by pseudopodia; *Entamoeba* means "internal amoeba", and its members are commensals or intestinal parasites.
• *Entamoeba histolytica* lives in the large intestine and has two forms: the trophozoite (active) and the cyst (quiescent). The cyst is the infective form and survives weeks in the environment; transmission is faecal-oral, from cysts on fruit and vegetables and in contaminated water.
• Excystation in the intestine gives eight trophozoites from one quadrinucleate cyst; encystation returns the parasite to the quadrinucleate cyst that leaves in the stool.
• Most infected people are asymptomatic carriers. Amoebiasis (amoebic dysentery) gives abdominal pain, bloody diarrhoea and intestinal lesions, because the parasite secretes proteases, enters the epithelium, feeds on blood cells, and spreads through the bloodstream to the liver, lungs and brain, where abscesses are fatal without treatment. More than 55,000 deaths a year.
• Morphology: the trophozoite is amoeboid and contains ingested erythrocytes, which distinguish it from the commensal species; the mature cyst is spherical with 4 nuclei and a chromatoid body of aggregated ribosomes; chromatin lies at the nuclear periphery and in a central karyosome.
• E. histolytica carries recombination genes and may undergo meiosis in the conversion of the uninucleate trophozoite into the quadrinucleate cyst, but the sexual process has not been studied.
• The other human species are nonpathogenic: *E. dispar* (like histolytica but never invades), *E. coli* (barely moves, stays round) and *E. gingivalis* (in the gingival pockets, in 95% of people with gum disease and 50% with healthy gums, spread by kissing and shared utensils). All of them have trophozoite and cyst forms.