Entamoeba histolytica (Gr. entos - within : amoiba - change histos - tissues ; lysis - dissolve) is a microscopic and monogenetic parasite that inhabits the large intestine and causes amoebic dysentery or amoebiasis in man.
It is cosmopolitan in distribution but more common in the tropical and subtropical regions of the world. It is common in the people of rural and densely populated urban areas wherever the hygienic conditions are poor.
Structure : Entamoeba histolytica passes through three distinct stages in its life cycle namely
(i) Trophozoite stage
(ii) Precystic stage &
(iii) Cystic stage
(i) Trophozoit stage : It is the most active, motile, feeding and pathogenic stage that lives in the mucosa and sub-mucosa membrane of the large intestine. It moves with the help of a lobopodium which is produced anteriorly. The body of the trophozoite is surrounded by plasma- lemma. Its cytoplasm is differentiated into outer clear, viscous non-granular ectoplasm and inner fluid like, granular endoplasm. Ribosomes, food vacuoles and vesicular, cartwheel shaped nucleus are present in the endoplasm. The absence of mitochondria indicates the obligate anaerobic nature of Entamoeba histolytica. It produces the proteolytic enzyme called histolysin due to which the species name histolytica was assigned to it. Due to the effect of this enzyme the mucosa and sub-mucosa of the gut wall are dissolved releasing some amount of blood, tissue debris which are ingested by the trophozoites. Hence the food vacuoles are with erythrocytes fragments of epithelial cells and bacteria. Presence of RBC in food vacuoles and cartwheel shaped nucleus are the characteristic features of the trophozoites of Entamoeba histlytica.
(ii) Precystic stage : It is the non-feeding and non-pathogenic stage of Entamoeba histolytica that is found in the lumen of large intestine.
It is a small, spherical or oval, non-motile form. The cytoplasm of the precystic stage stores glycogen granules and chromotoid bars (made of ribonucleo protein) which act as reserve food.
(iii) Cystic stage : It is round in shape and is surrounded by a thin, delicate and highly resistant cyst wall. It is found in the lumen of the large intestine.
The process of development of cyst wall is called encystation. Which is a means of tide over the un- favourable conditions that the parasite is going to encounter while passing to a new host. Soon after the encystation, the nucleus undergoes two successive mitotic divisions to form four daughter nuclei. This type of cystic stage is called tetranucleate cyst or mature cyst which is the stage infective to man.
Life cycle : The trophozoites undergo binary fissions in the wall of the large intestine and produce a number of daughter entamoeba. They feed upon the bacteria and the host's tissue elements, grow in size and again multiply. After repeated binary fission some of the young ones enter of the lumen of the large intestine and transform into precystic stages. Here, the precystic stages transform into cystic stages which in turn develop into tetranucleate cysts. The entire process is completed only in a few hours. These tetranucleate cysts come out along with the faecal matter and can remain alive for about 10 days. The cyst reach new host through contaiminated food and water. In the small intestine of a new human host the cyst wall gets reptured releasing the tetranucleate amoebae. Such tetranucleate excystic amoebae are called metacysts.
The four nuclei of the metacyst undergo mitotic divisions and produce eight nuclei. Each nucleus gets a bit of the cytoplasm and thus eight daughter entamoebae or metacystic trophozoites are produced. These young ones develop into feeding stages called trophozoites. They invade the mucous membrane of the large intestine and grow into mature trophozoites.
