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It's microscopic, blob-shaped and toothless. But the amoeba known as Entamoeba histolytica turns out to be the werewolf of parasites.

New research shows that this nearly transparent microbe, a significant scourge in parts of the developing world, carves chunks off a living cell and devours them, leading to the cell's disintegration and death. Never before have scientists discovered an organism that kills cells by biting. The finding could lead to new medications to treat the dysentery and diarrhea caused by the amoeba or to vaccines to prevent infection.

"I've spent my entire career studying this parasite … and I didn't realize this was happening," says study co-author William Petri of the University of Virginia. The amoeba's brutal modus operandi is "something everyone had completely missed for literally 100 years."

Most people infected with the amoeba don't have symptoms, but anyone who has symptoms is likely to be very unhappy. The amoebas colonize the large intestine and bore their way into the intestinal wall, in some cases bursting all the way through the intestine and invading organs such as the liver. Victims may suffer anything from diarrhea to – in rare cases – death. Numbers are hard to come by, but one study of seven low-income countries found that the amoeba is one of the top 10 causes of moderate to severe diarrhea in young children.

Good sanitation and hygiene keep the amoeba from spreading widely in countries such as the United States, but it's a significant problem in India, Somalia and other developing nations. In one slum in Dhaka, Bangladesh, for instance, the amoeba infects one-third of children before their first birthday, according to a separate study by Petri and his colleagues. Infection is an important cause of childhood diarrhea, which can be deadly, and repeated infections can stunt a child's growth.

Researchers have known for more than a century that the amoeba kills human cells, but they weren't sure how it did the deed. So Petri's colleague Katherine Ralston examined a mixture of amoebas and human cells under a microscope. Ralston saw that when an amoeba attached to a living cell, the parasite swiftly began gobbling chunks of its victim's outer covering and innards, inflicting fatal damage. When Ralston added amoebas to a sample of mouse intestines, she saw the parasites nibbling at the intestinal cells before invading the gut tissue. The amoeba "bites" by engulfing a chunk of its victim and pinching it off from the main cell.

"The intestine is a good enough barrier to keep out all the bacteria … but here you have this amoeba chewing and making its way into the gut quite rapidly," Ralston says. "It was pretty surprising to see first-hand just how well they get into the tissue."

The study, published in this week's Nature, shows that, at least in a laboratory dish, the amoeba augers holes in the large intestine in part by nibbling its way into the tissue. The researchers also found that it's possible to hold the amoeba at bay by adding substances that gum up the parasite's cell-biting machinery. The discovery offers a new target for anti-amoeba medications to supplement today's drugs, which work well but may eventually develop resistance.

The study is a "technical tour de force," says Christopher Huston of the University of Vermont College of Medicine, who studies intestinal parasites and was not involved in the new research but has worked with Petri. How the amoeba kills cells "has been really elusive. (The new work) gives a lot of insight into how that's occurring, which is a nice advance."

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