Entamoeba histolytica : Morphology, Life Cycle, Culture, Pathogenicity
- Entamoeba histolytica is an anaerobic parasitic amoebozoa.
- Entamoeba histolytica is found in the human colon. It commonly occurs in the lumen of the colon as a commensal, but sometimes invades the intestinal tissues to become a pathogen.
- Entamoeba histolytica is world-wide in prevalence. It is much more common in the tropics than elsewhere, but it has been found wherever sanitation is poor, in all climatic zones, from Alaska (61° N) to the Straits of Magellan (52°S).
- While the large majority of the infected are asymptomatic, invasive amoebiasis causes disabling illness in an estimated 50 million persons and death in 50,000 annually, mostly in the tropical belt of Asia, Africa and Latin America.
- It is the third leading parasitic cause of mortality, after malaria and schistosomiasis.
Entamoeba histolytica occurs in three forms—the trophozoite, precystic and cystic stages .
- The trophozoite or the vegetative form is the growing or feeding stage of the parasite.
- It is irregular in shape and varies in size from about 12 to 60 μm.
- It is large and actively motile in freshly passed dysenteric stools, while in convalescents and carriers, it is much smaller. The parasite as it occurs free in the lumen as a commensal is generally smaller in size, about 15 to 20 μm and has been called the minuta form.
- The trophozoites from acute dysenteric stools often contain phagocytosed erythrocytes. This feature is diagnostic as phagocytosed red cells are not found in any other commensal intestinal amoebae.
- The trophozoite divides by binary fission once in about 8 hours.
- Trophozoites are delicate organisms and are killed by drying, heat and chemical disinfectants, They do not survive for any length of time in stools outside the body. Therefore, the infection is not transmitted by trophozoites. Even if live trophozoites from freshly passed stools are ingested, they are rapidly destroyed in the stomach and cannot initiate infection.
2. Precystic Stage
- Some trophozoites undergo encystment in the intestinal lumen. Encystment does not occur in the tissues nor in feces outside the body.
- Before encystment the trophozoite extrudes its food vacuoles and becomes round or ovoid about 10 to 20 μm in size. This is the precystic stage of the parasite. It secretes a highly refractile cyst wall around it and becomes the cyst.
3. Cystic Stage
- The cyst is spherical, about 10 to 20 μm in size.
- The early cyst contains a single nucleus and two other structures—a mass of glycogen and one to four chromatoid bodies or chromidial bars, which are cigar-shaped or oblong refractile rods with rounded ends. The chromatoid bodies are so called because they stain with haematoxylin like chromatin.
- As the cyst matures, the glycogen mass and chromidial bars disappear. The nucleus undergoes two successive mitotic divisions to form two and then four nuclei. The mature cyst is quadrinucleate.
The infective form of the parasite is the mature cyst passed in the feces of convalescents and carriers. The cysts can remain viable under moist conditions for about ten days.
- The cysts ingested in contaminated food or water pass through the stomach undamaged and enter the small intestine. When the surrounding medium becomes alkaline.
- The cyst wall is damaged by trypsin in the intestine, leading to excystation. The cytoplasm gets detached from the cyst wall and amoeboid movements appear causing a tear in the cyst wall through which the quadrinucleate amoeba emerges. This stage is called the metacyst. The nuclei in the metacyst immediately undergo division to form eight nuclei, each of which gets surrounded by its own cytoplasm to become eight small amoebulae or metacystic trophozoites.
- If excystation takes place in the small intestine, the metacystic trophozoites do not colonise there, but are carried to the caecum.
- The optimum habitat for the metacystic trophozoites is the caecal mucosa where they lodge in the glandular crypts and undergo binary fission. Some develop into precystic forms and cysts, which are passed in feces to repeat the cycle.
- The entire life cycle is thus completed in one host.
Infection with E. histolytica does not necessarily lead to disease. Infact, in most cases it remains within the lumen of the large intestine, feeding on the colonic contents and mucus as a commensal without causing any ill effects. Such persons become carriers or asymptomatic cyst passers, as their stool contains cysts. They are responsible for the maintenance and spread of infection in the community.
The infection may get spontaneously eliminated in many of them. Sometimes, the infection may be activated and clinical disease ensues. Such latency and reactivation are characteristic of amoebiasis.
- Robinson’s medium has been widely used for cultivation of amoebae. In these media and their modifications, amoebae grow only in presence of enteric bacteria or other protozoa and starch or other particles.
- Axenic cultivation which does not require the presence of other microorganisms or particles, was first developed by Diamond in 1961. This yields pure growth of the amoeba and has been very useful for physiological, immunological and pathogenicity studies of amoebae.
- Intestinal Amoebiasis, Amoebic dysentery
- Extraintestinal (Invasive) Amoebiasis such as Hepatic amoebiasis, Cutaneous amoebiasis, Pulmonary amoebiasis