Amoebas are fascinating single-celled organisms. They typically move about by having their internal fluid (their cytoplasm) flow around and they eat their food by surrounding and digesting it with their body, in a process called phagocytosis. Some amoeba are parasites, such as Entamoeba histolytica, which can infect people and cause a potentially fatal diarrheal disease called amoebioasis that’s thought to kill about 70,000 people each year.
The E. histolytica parasite infects the gastrointestinal tract, where it can cause devastating tissue damage. But how exactly the amoebas kill those cells has been unclear — the most likely explanation has been that the amoebas secrete toxins to kill the cells, and, once dead, the cells get digested by the amoebas. However, a recent study revealed what’s actually happening — the amoebas are eating the cells alive!
Based on detailed images of the amoebas in action, the researchers could see that the amoebas take bites out of the cells until they die from the damage. Once they’ve killed the cells, the amoebas stop ingesting them. Because the amoebas reject cell corpses, it suggests that this method of eating and killing the host cells may not serve a significant nutritional role. Instead, the authors suggest that this tactic is better suited for invading the host — when host cells are so tightly packed together that engulfing one whole would be difficult (such as in epithelial tissue), this approach makes sense.
How these amoeba take bites out of living cells is similar to a process known as trogocytosis. Trogocytosis is carried out by certain white blood cells when they bind and remove the surface molecules from antigen-presenting cells. The white blood cells put these surface molecules back on their own surface. But, unlike E. histolytica, the white blood cells don’t kill the cells they take the molecular pieces from. E. histolytica is the only parasite known to use a process like trogocytosis.
So while we like to classify different organisms based on their typical behaviors and characteristics, it’s important to always closely examine each critter individually to make sure we’re not overlooking a key difference that could help us better understand them and develop a more effective medical treatment.
For further reading:
- Katherine S. Ralston et al.’s article “Trogocytosis by Entamoeba histolytica contributes to cell killing and tissue invasion”
- Ed Yong’s article “Amoeba Eats Cells Alive”
- Teisha J. Rowland’s book Biology Bytes: Digestible Essays on Stem Cells and Modern Medicine