page_title: Balantidium coli
section_title: Life cycle
hierarchical_section_title: Balantidium coli / Life cycle
caption_attribution_description: Cysts are the parasite stage responsible for transmission of balantidiasis . The host most often acquires the cyst through ingestion of contaminated food or water . Following ingestion, excystation occurs in the small intestine, and the trophozoites colonize the large intestine . The trophozoites reside in the lumen of the large intestine of humans and animals, where they replicate by binary fission, during which conjugation may occur . Trophozoites undergo encystation to produce infective cysts . Some trophozoites invade the wall of the colon and multiply. Some return to lumen and disintegrate. Mature cysts are passed with feces .
context_page_description: Balantidium coli is a parasitic species of ciliate alveolates that causes the disease balantidiasis. It is the only member of the ciliate phylum known to be pathogenic to humans.
context_section_description: Infection occurs when a host ingests a cyst, which usually happens during the consumption of contaminated water or food. Once the cyst is ingested, it passes through the host’s digestive system.
  While the cyst receives some protection from degradation by the acidic environment of the stomach through the use of its outer wall, it is likely to be destroyed at a pH lower than 5, allowing it to survive easier in the stomachs of malnourished individuals who have less stomach acid. Once the cyst reaches the small intestine, trophozoites are produced. The trophozoites then colonize the large intestine, where they live in the lumen and feed on the intestinal flora. Some trophozoites invade the wall of the colon using proteolytic enzymes and multiply, and some of them return to the lumen. In the lumen, trophozoites may disintegrate or undergo encystation. Encystation is triggered by dehydration of the intestinal contents and usually occurs in the distal large intestine, but may also occur outside of the host in feces. Now in its mature cyst form, cysts are released into the environment where they can go on to infect a new host.
caption: Balantidium coli life cycle
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