Entamoeba histolytica : Morphology, Life Cycle, Classification, Diagram, Culture, Pathogenicity, Diseases, & Treatment
- Entamoeba histolytica is an anaerobic parasitic amoebozoa. It is found in the human colon.
- Infection with E. histolytica does not necessarily lead to disease. Mostly, it remains within the lumen of the large intestine as a commensal without causing any ill effects (Carriers or asymptomatic cyst passers). 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.
- Entamoeba histolytica is world-wide in prevalence.
- 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 .
1. Trophozoite (Vegetative form)
- Trophozoite: Growing or feeding stage of the parasite.
- Shape: Irregular.
- Size: 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 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.
- Division: Binary fission once in about 8 hours.
- Environment withstand: Highly delicate thus, 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
- Cyst shape: Spherical.
- Size: About 10 to 20 μm.
- Mature cyst : Infective form of the parasite
- Immature cyst: Mononucleate. The early cyst contains a single nucleus and two other structures—a mass of glycogen and one to four chromatoid bodies or chromidial bars (cigar-shaped or oblong refractile rods with rounded ends). The chromatoid bodies are so called because they stain with haematoxylin like chromatin.
- Mature cyst: Quadrinucleate. 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 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.
- Cysts ingestion through contaminated food or water → Stomach (remain undamaged) → Small intestine → Excystation (damaged by trypsin) → Cytoplasm detachment from the cyst wall and amoeboid movements appear → Tear in the cyst wall through which the quadrinucleate amoeba emerges (metacyst) → Metacyst nuclei divides → 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.
- Optimum habitat for the metacystic trophozoites: Caecal mucosa where they lodge in the glandular crypts.
- Trophozoites → 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.
- Robinson’s medium: Amoebae grow only in presence of enteric bacteria or other protozoa and starch or other particles.
- Axenic cultivation: It does not require the presence of other microorganisms or particles. 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