Life Cycle of Balantidium coli
Balantidium coli has 2 developmental stages: a trophozoite
stage and a cyst stage. (See Morphology and Transmission for further descriptions of the
stages.)
The
cyst is the infective stage of Balantium coli life cycle. Once the cyst is ingested via
feces-contaminated food or water, it passes through the host digestive
system. The tough cyst wall allows the
cyst to resist degradation in the acidic environment of the stomach and the
basic environment of the small intestine until it reaches the large
intestine. There, excystation takes
place. Excystation produces a trophozoite from the cyst stage.
The
motile trophozoite then resides in the lumen of the large intestine, feeding on
intestinal bacterial flora and intestinal nutrients. Trophozoites multiply by asexual binary fission or sexual
conjugation (with the exchange of nuclear material). The trophozoite may become invasive and penetrate the mucosa of
the large intestine (See Clinical
Presentation). Trophozoites are
released with the feces, and encyst to form new cysts. Encystation takes place in the rectum of the
host as feces are dehydrated or soon after the feces have been excreted.
Cysts
in the environment are then ready to infect another host.
Excystation
in large intestine
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