Entamoeba histolytica and Amoebiasis

Entamoeba histolytica and Amoebiasis

Entamoeba histolytica is a microscopic, unicellular protozoan parasite that lives in the large intestine (colon) of humans and is the causative organism of amoebiasis, commonly known as amoebic dysentery.

Structure of the trophozoite

The trophozoite is the actively feeding, actively motile and pathogenic form of the parasite. It is a naked, colourless mass of cytoplasm differentiated into a clear outer ectoplasm and a granular inner endoplasm. Locomotion and feeding are carried out by pseudopodia — finger-like projections thrown out from the ectoplasm. The endoplasm contains a single spherical nucleus with a small central karyosome, food vacuoles (which in a pathogenic, invasive trophozoite frequently contain engulfed host red blood cells — a key diagnostic feature), and no permanent organelles for respiration.

An obligate anaerobe

Entamoeba histolytica is described as an obligate anaerobe because it lives inside the oxygen-poor environment of the large intestine and lacks mitochondria; it derives its energy exclusively through anaerobic metabolic pathways and cannot survive in the presence of free oxygen. This anaerobic lifestyle is one reason the parasite is confined to the gut lumen and gut wall rather than circulating in oxygenated blood.

Reserve food

Instead of storing energy as starch (as plants do) or fat, Entamoeba histolytica stores its reserve food material as glycogen, granules of which are visible in the endoplasm, particularly within the cyst stage, where they provide the energy needed to survive outside a host.

Life cycle stages

The life cycle of Entamoeba histolytica is simple and direct, involving only one host (humans) and no intermediate host or vector, though flies and contaminated water/food act as passive carriers of the infective stage.

1. Trophozoite (metacystic trophozoite) — infection begins when a person swallows the mature, infective quadrinucleate cyst in contaminated food or water. Digestive juices in the small intestine dissolve the cyst wall (excystation), releasing a single amoeba that immediately divides to give rise to eight small, uninucleate metacystic trophozoites. These migrate to the large intestine.
2. Trophozoite multiplication — in the colon, the metacystic trophozoites grow, feed on bacteria and host-cell debris, and multiply repeatedly by simple binary fission, colonising the mucosal surface.
3. Precystic stage — as conditions in the gut change (for example, dehydration of gut contents or an unfavourable environment), an actively feeding trophozoite stops moving, egests any undigested food, rounds off and shrinks slightly to become a precyst.
4. Cystic stage — the precyst secretes a tough, protective cyst wall around itself. The single nucleus inside then undergoes two successive mitotic divisions to produce the mature, resistant, thick-walled quadrinucleate cyst, which is passed out of the body along with the faeces and can survive for days to weeks in a moist external environment, ready to infect a new host.

Pathogenicity

Under normal, non-invasive conditions many infected people harbour Entamoeba histolytica in the lumen of the colon as harmless commensal trophozoites feeding on bacteria — this is called primary (luminal) amoebiasis, and such individuals, who show no symptoms yet continuously shed infective cysts in their stool, are known as asymptomatic cyst passers or carriers, and are epidemiologically important because they silently spread the infection. …