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Amoeba’s deadly nibbling of human cells caught on camera

LONDON — The gruesome means by which a parasite wreaks havoc in the human body has been laid bare for the first time by researchers who filmed the bug in action at the end of a microscope.

A screen capture of the video taken by University of Virginia scientists showing the parasite Entamoeba histolytica destroying a human cell.

A screen capture of the video taken by University of Virginia scientists showing the parasite Entamoeba histolytica destroying a human cell.

LONDON — The gruesome means by which a parasite wreaks havoc in the human body has been laid bare for the first time by researchers who filmed the bug in action at the end of a microscope.

Infections of Entamoeba histolytica can trigger intestinal ulcers, gut inflammation and life-threatening diarrhoea in children in developing countries, but how the organism caused such distress was unknown.

One theory was that the parasite released toxins that destroyed cells in the intestine, where it hunkers down in people who carry the infection. However, the high-magnification video footage drew a thick line through that idea.

“To our surprise, with live microscopy, we found that the parasites were taking bites out of the human cells and that this eventually led to their death,” said Dr Katherine Ralston at the University of Virginia.

“This is the first demonstration that nibbling can be used to kill,” she told the Guardian.

The same modus operandi might be employed by the brain-eating amoeba Naegleria fowleri, Dr Ralston said.

Amoeba attacks on human cells began almost as soon as Dr Ralston introduced them to one another in a dish. The human cells took some chomping to kill them off, surviving for about 10 minutes before their membranes were torn apart and their inner contents spewed out.

“It’s the accumulation of physical damage. They can survive a limited number of bites, but as the amoeba takes more and more bites, eventually the cell loses its membrane integrity and dies,” Dr Ralston said. Details of the study are reported in the scientific journal Nature.

Amoeba do not have mouths to speak of, but the parasites appeared to use their cytoskeleton — the scaffolding that maintains their shape — to generate enough force to take chunks out of nearby cells.

“We think it’s really critical for the parasite to tear a bite off another living cell. The human cell material is stretched and then sheared off into individual bites,” she said. The process is called trogocytosis, after the Greek word trogo, meaning nibble.

The amoeba did not finish the human cells off once they had killed them, but left their ripped-up remains where they fell and moved on to the next cell.

Other amoeba who encountered killed human cells probed them for a bit, but moved on instead of taking a bite themselves.

Professor William Petri, a senior author on the paper, said the Virginia group was working on a vaccine to protect children against the infection. One target that has come from Dr Ralston’s work is a molecule on the surface of the parasite that, when blocked, prevents the amoeba from being able to nibble.

Records from children born in an urban slum in Dhaka, Bangladesh, show that one in three is infected with the parasite by the time they reach 12 months. The organism, which spreads through contaminated food and water, kills about 70,000 people each year, said the World Health Organization.

Some questions that remain are why the parasites stop eating once they have killed a human cell, as well as how they and others in the colony know when the job is done. Dr Ralston says the parasites might be picking up on substances that are only exposed when a cell is pulled wide open.

Prof Petri speculates that there might be a good reason for the parasites leaving scraps of dead cells behind. In an infected person, the remains would probably be cleared from the body by cells called macrophages, which release anti-inflammatory chemicals.

That suppression of the immune system might help the parasites to survive in humans for long periods. People who catch the infection can be healthy for six months, but later develop a liver abscess caused by the parasite, Prof Petri said. The GUARDIAN

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