Most of us have seen, through a microscope, amoebas, those tiny, one-cells that flow about, changing shape. But did you know you have a number of them inside your body?

There are several species of amoeba which are commensal or symbiotic with us. This article will not deal with the numerous species whose main business is to cause human disease. These are the harmless folks. (Usually.) This is by no means a complete list! There are a lot of these guys!

  • The mouth amoeba, Entamoeba gingivalis, lives in your mouth, and is passed directly from person to person in kissing and in sharing food. It does not form cysts as most human amoebas do. Nearly everyone has them. They feed on loose cells and debris in the mouth.
  • The intestinal amoeba, Entamoeba coli, lives in the large intestine, but lives only on the food there, not on your own cells. It sometimes eats parasites like Giardia, thus doing you a favor. This one will encyst, pass out of your intestine, and wait for someone else to eat food contaminated with human waste, or drink contaminated water. The cyst can survive the acid in the stomach, so that the amoeba ends up where it wants to be in this next person. Unlike many organisms which follow this route, though, it will do you no harm.
  • The dysentery amoeba, Entamoeba histolytica, is a shifty fellow, though. Most of the time it's perfectly harmless, and about 10% of the human population carries it around all the time, but under the right circumstances - usually some weakness in the immune system, but there may be other reasons - it can cause serious or fatal illness by invading the intestinal wall and ingesting host tissues. It may also spread to the liver and other organs. It too, like Entamoeba coli, is transmitted in encysted form with contaminated food or water.

Kingdom Protista, Sarcondines.
Authorities differ very much in the classification of Protista, some ranking them in classes, some in numerous phyla. Since amoebas do not "breed", but rather reproduce by fission, the concept of species is a difficult one to apply in their case anyway.

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