Typical life cycle of nonpathogenic intestinal amebae
Entamoeba coli, Ent. hartmanni, Ent. polecki, Endolimax nana, and Iodamoeba buetschlii are generally considered nonpathogenic and reside in the large intestine of a human host (A). Both cysts and trophozoites of these species are passed in stool and considered diagnostic (1). Cysts are typically found in formed stool, whereas trophozoites (the active stage) are typically found in diarrheal stool. Colonization of the nonpathogenic amebae occurs after ingestion of mature cysts in fecally-contaminated food, water, or fomites (i.e., inanimate objects or substances capable of transferring pathogens) (2). Excystation (release of the trophozoite from the cyst) occurs in the small intestine (3) and the trophozoites migrate to the large intestine. The trophozoites multiply by binary fission and both trophozites and cysts are passed in the feces (1). Because of the protection conferred by their cell walls, the cysts can survive days to weeks in the external environment and are responsible for transmission. Trophozoites passed in the stool are rapidly destroyed once outside the body and if ingested would not survive the gastric environment.
From Centers for Disease Control Parasites and Health website.