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Thirty years after scientists had formulated the term "hygiene hypothesis" to suggest that increased exposure to microorganisms could be beneficial to health, researchers at the University of Colorado at Boulder have identified an anti-inflammatory fat in a soil-dwelling bacterium that could be responsible for it.
The discovery, published Monday in the newspaper psychopharmacologycan at least partly explain how Mycobacterium vaccae bacteria suppresses stress-related disorders. This also brings researchers closer to the development of a "stress vaccine" based on microbes.
"We think that a special sauce is causing the protective effects of this bacteria, and this fat is one of the main ingredients of this special sauce," said Christopher Lowry, lead author and author. professor of integrative physiology.
The British scientist David Strachan had for the first time proposed the controversial "hygiene" hypothesis in 1989, suggesting that in our modern and sterile world, the lack of exposure to microorganisms in the United childhood resulted in impaired immune system and higher rates of allergies and asthma.
Researchers have since refined this theory, suggesting that it was not a lack of exposure to the pathogens involved, but rather "old friends" – beneficial microbes in soil and the environment – and that mental health was affected.
"The idea is that since humans have moved away from farms and agriculture and hunter-gatherers in cities, we have lost contact with organisms that regulate our immune system and remove inappropriate inflammation, "said Lowry, who prefers these expressions. badumption of old friends "or" farm effect ". "This put us at a higher risk of inflammatory disease and stress-related psychiatric disorders."
Lowry has published numerous studies demonstrating a link between exposure to healthy bacteria and mental health.
One of them showed that children raised in rural areas, surrounded by bacteria-laden animals and dusts, develop an immune system that is more resistant to stress and is less likely to suffer from mental illness than city dwellers without pets.
Others have shown that when a particular bacterium, Mycobacterium vaccae, is injected into rodents, it alters the behavior of animals in the same way as antidepressants and has lasting anti-inflammatory effects on the brain . Studies suggest that an exaggerated inflammation increases the risk of disorders related to trauma and stressors, such as post-traumatic stress disorder (PTSD).
A recent study by Lowry, published in the Proceedings of the National Academy of Sciences in 2017, showed that injections of M. vaccae before a stressful event could prevent an "ESPT" -like syndrome in mice, battling Stress-induced colitis and make animals act less anxious when they are stressed again.
"We knew it worked, but we did not know why," Lowry said. "This new document helps clarify that."
For the new study, Lowry and his team identified, isolated and chemically synthesized a new fatty acid called 10 (Z) -hexadecenoic acid found in Mycobacterium vaccae and used next-generation sequencing techniques to study its interaction with macrophages. or immune cells, when the cells have been stimulated.
They discovered that inside the cells, the lipid acted as a key in a lock, binding to a specific receptor, the peroxisome proliferator-activated receptor (PPAR), and inhibiting a large number key pathways responsible for inflammation. They also found that when the cells were pretreated with the lipid, they were more resistant to inflammation when they were stimulated.
"It seems that these bacteria that we've evolved have a knack for," Lowry said. "When they are absorbed by the immune cells, they release those lipids that bind to that receptor and block the inflammatory cascade."
Lowry has long been considering developing a M. vaccae "stress vaccine" that could be administered to first responders, soldiers and others in high-stress jobs to help protect them from psychological harm. by stress.
"It's a huge step forward for us as it identifies an active component of the bacterium and the receptor of that active component in the host," he said.
Simply knowing the mechanism by which M. vaccae is reaping benefits could enhance confidence in him as a potential treatment. And if other studies show that new fat alone has therapeutic effects, this molecule could become a target for drug development, he said.
Overall, the study offers further evidence that our "old friends" have a lot to offer.
"It's only one strain of a kind of bacterium that's in the soil, but there are millions of other strains in the soil." "said Lowry. "We are just beginning to see the tip of the iceberg in terms of identifying the mechanisms by which they have evolved to keep us healthy. That should inspire us with respect."
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