FLOW CHART OF ENTAMOEBA HISTOLYTICA
The life cycle of Entamoeba histolytica, a parasitic amoeba, in humans is a complex process that involves several stages. Entamoeba histolytica is responsible for a disease called amoebiasis, which can cause severe gastrointestinal symptoms. E. histolytica is a monogenetic parasite. The infection begins when the host swallows the mature quadrinucleate cyst along with the contaminated food or water. As the cyst wall is resistant to the acidic content of the stomach, the quadrinucleate cyst passes unaltered into the small intestine where excystation takes place. In the intestine, the cyst wall is digested by the action of trypsin in an alkaline medium at a temperature of 37°C. During this process, the cytoplasmic body retracts and loosens from the cyst wall. Pseudopodia are formed at various points and vigorous amoeboid movements occur within the cyst. Frequently, the pseudopodia press against the wall at certain spots as though the imprisoned organism were searching for exit. Eventually, a tetranucleate amoeba known as metacyst (amoeba with four nuclei) emerges out. Immediately on emergence, the four nuclei of the metacyst undergo division to form eight nuclei. Each nuclei gets surrounded by a bit of its own cytoplasm and leads an independent existence. Thus, eight amoebulae are formed. These are known as metacystic or metacyclic trophozoites which are actively motile. The metacystic trophozoites move down to the caecum and ileocloacal region of the intestine. The young amoebulae being actively motile invade the tissues and finally lodge themselves in the mucosa and submucosa of the large intestine-its final abode. They prefer this site as the organic material (food), pH and gases in this part of the large intestine are more stable and ideal for the existence of E. histolytica trophozoites. Here, the trophozoites grow at the expense of living tissues and multiply by simple binary fission. The trophozoites secrete histolysin which causes necrosis and destruction of the host’s tissue and helps the parasite to derive nourishment from the dissolved dead tissues.
However, there are some non-invasive trophozoites that remain in the lumen of the large intestine and multiply by binary fission. These trophozoites feed on the hosts nutrients from the surrounding medium. Some of the trophozoites from the cells of the mucosa and submucosa after repeated binary fission move back into the lumen of the intestine.
When the conditions become unfavourable for the trophozoites in the lumen of the large intestine, they start to develop a cyst wall. A pre-cyst is first formed which soon becomes a uninucleate immature cyst. The nucleus within the cyst divides to first form a binucleate cyst. The nucleus divides again for the second time to form a quadrinucleate/tetranucleate mature cyst (infective stage). The transformation of trophozoite into a mature quadrinucleate cyst is called encystations and is a means of protection of a species from extinction. Encystation does not take place in the tissues of man: neither in the intestinal mucosa nor in the liver, lungs etc. Thus, the metastatic invasion of the trophozoite for all biological purposes is a dead end for the parasite.
Encystation takes only a few hours and the mature quadrinucleate cyst can remain able to exist in the intestinal cavity of the large intestine for only two days. Mature quadrinucleate cysts are passed out along with the faeces of the host. About 45 million cysts may be expelled out from one infective person in a day. The cysts are resistant to the environmental conditions and can live for a few weeks to a few months depending on the temperature (thermal death occurs at 50°C). Moisture is essential for the long existence of the cyst. They can live upto 10 days in a moist stool.
Trophozoites are also expelled out along with the cysts in the faeces, but they cannot survive outside the body of the host for more than an hour and even if they are ingested by another human being during this period they are killed in the body of the host by the acidic juices of the stomach.
A very important point to note is that both excystation and encystation are not reproductive processes.
Encystation and excystation can take place in the same host; another host is required only for the continuation of the species.