Brain diseases are linked to intestinal bacteria – 01/03/2019

By CARL ZIMMER

In 2014, John Cryan, a professor at University College Cork, Ireland, attended a meeting in California on Alzheimer's disease. He was not an expert in dementia. Instead, I studied the microbiome, the billions of microbes in the human body in good health.

Cryan and other scientists were beginning to find indications that these microbes could influence the brain. Perhaps, he said at the scientific meeting, the microbiome has a role in Alzheimer's disease.

The idea was not well received.

"I had never given the floor to as many people as I did not believe what I was saying," recalls Cryan.

Since then, many things have changed: scientists are discovering that the microbiome may play a role not only in Alzheimer's disease, but also in Parkinson's disease, depression, schizophrenia, autism and other conditions.

Sangram Sisodia, a neurobiologist at the University of Chicago, was one of the skeptics of this meeting. The doctor decided to submit the idea to a simple test.

He and his colleagues gave antibiotics to mice that could develop a version of Alzheimer's disease to eradicate much of the intestinal bacteria in mice. Later, when scientists inspected the animals' brains, they found far fewer lumps of protein badociated with dementia.

Sisodia now suspects that only a few species in the intestines – perhaps only one – influence the evolution of Alzheimer's disease, perhaps by releasing a chemical that changes how immune cells function in the brain.

Traditionally, microbiologists paid little attention to the brain because there did not seem to be many cases. The brain is protected from microbial invasion by the so-called blood-brain barrier. Usually only small molecules can pbad.

"Until now, in 2011, it was considered crazy to seek badociations between the microbiome and the behavior," said Rob Knight, a microbiologist at the University of California at San Diego.

Different studies have revealed surprising trends. Autistic children have unusual patterns of microbial species in their stool. Differences in intestinal bacteria have also been reported in people with other mental disorders.

However, none of these badociations demonstrate cause and effect. Finding an unusual microbiome in people with Alzheimer's disease does not mean that the bacteria is driving the disease.

Fecal transplants can help identify these links. As part of their research on Alzheimer's disease, Sisodia and colleagues transferred feces from common mice to mice treated with antibiotics. Once their microbiomes were restored, the antibiotic-treated mice again began to develop protein mbades.

"We are extremely convinced that it is the bacteria that are at the origin of this situation," he said.

Because researchers are transferring hundreds of species of bacteria at a time, the experiments can not reveal what are the causes of changes in the brain. Now researchers are locating the different strains that seem to be having an effect.

Mauro Costa-Mattioli and colleagues at Baylor College of Medicine, Texas, they studied mice that show symptoms of autism. A genetic mutation can prevent mice from coming into contact with other mice, for example.

When the researchers badyzed the microbiomes of these mice, they found that the animals did not have any common species called Lactobacillus reuteri. When they added a strain of this bacteria to the diet, the animals became social again.

Costa-Mattioli has found evidence that L. reuteri releases compounds that send a signal to the nerve endings of the intestines. The vagus nerve sends these signals from the intestines to the brain, where they modify the production of a hormone that promotes social bonds.

It turns out that other microbial species also send signals along the vagus nerve. However, others communicate with the brain via the blood.

As researchers better understand the influence of the microbiome on the brain, they believe that doctors can use it to treat psychiatric and neurological problems.

Researchers must now reach a difficult balance. His experiments have been promising, but scientists do not want to encourage the idea that remedies based on the microbiome will come soon.

© 2019 the New York Times