Amoebae- Parasitic Protozoan
The amoebae of the genus Entamoeba vary in their biology. Entamoeba histolytica or the dysentery amoeba occurs as a parasite in the large intestine of man and monkeys. E. coli is a harmless commensal in the same location, but E. gingivalis lives in the mouth. E. histolytica has three distinct phases in its life cycle, viz., trophozoite, precystic stage and cyst. Trophozoite and cyst are the stages which are commonly demonstrated in the faeces. In the tissues only trophozoite stage occurs. The trophozoite is the actively feeding and dividing form. It is about 12-30 μm in diameter.
A number of red blood corpuscles may be seen in the food vesicles in various stages of digestion in cytoplasm. The trophozoite stage continues to divide by frequent repeated binary fissions in the lumen of the large intestine, as a result of which colonisation of the parasite occurs. Some of these amoebae may invade the intestinal mucosa and become tissue dwelling. It is these tissue invaders which produce amoebic ulcers in the intestinal wall that cause diarrhoea and dysentery.
Only those forms which live in the intestinal lumen are capable of producing cysts, which pass out of the host's faeces. The mature cyst has got four nuclei and is the infective stage of the parasite. When food or water contaminated with cyst containing faeces is ingested, man acquires the infection. The viable cysts pass via stomach into the intestine, the cyst wall gets dissolved with the action of intestinal secretions and a quadrinucleate amoeba emerges. This undergoes divisions as a result of which eight uninucleate amoebulae are formed. These small amoebae soon start a new cycle.