New research suggests that some of the dirt you may have eaten as a child could have contained a type of bacteria that strengthens the immune system. The study reveals the bacteria Mycobacterium vaccae can produce a unique anti-inflammatory fatty acid with anti-inflammatory properties.

Back in the late 1980s a British scientist put forth a hypothesis that the increasing rates of allergies and asthma in modern, Western society could be attributed to growing obsessions with hygiene. The idea was dubbed the "hygiene hypothesis" and it caught on in many alternative circles. The implication was that exposure to certain infectious agents was good for the immune system.

Criticism of the hypothesis swiftly arose, with some scientists calling the idea misleading and claiming there was no evidence that good hygiene disrupts a person's developing immune system. The idea subsequently evolved into what some called the "old friends" hypothesis. This posits the idea that excessive hygiene practices reduce exposure to beneficial bacteria, which is what ultimately impairs the immune system.

Christopher Lowry, from the University of Colorado Boulder, has been investigating the "old friends" hypothesis for several years, specifically researching how exposure to particular bacteria can improve our immune system and lower the risk of developing stress-related forms of mental illness.

"The idea is that as humans have moved away from farms and an agricultural or hunter-gatherer existence into cities, we have lost contact with organisms that served to regulate our immune system and suppress inappropriate inflammation," explains Lowry. "That has put us at higher risk for inflammatory disease and stress-related psychiatric disorders."

One of Lowry's most compelling current research focuses is a bacterium called Mycobacterium vaccae. First discovered in the 1990s in soil near the shores of Lake Kyoga in Uganda, the bacteria was initially investigated for its impressive immune system modulating properties. Subsequent studies in animals have discovered it can confer impressive antidepressant effects.

A 2016 study Lowry worked on revealed that, not only did the bacteria seem to prevent the onset of anxiety or fear-like behaviors in mouse experiments, but it also potentially acted as a protective agent when administered prior to stressful events. When mice were injected with the bacteria before being subjected to a traumatic scenario, the animal was less likely to develop a PTSD-like syndrome. It was almost as if the bacteria had the ability to function like a stress vaccine, lowering the long-term repercussions of exposure to a stress-related event.

How this could be happening though remained a mystery. Lowry's work last year led him and his team to discover the bacteria can induce an anti-inflammatory state in the brain. It was this anti-inflammatory action they hypothesized as conferring the beneficial mental health protections, however it was still unclear exactly how this was happening.

The incredible new breakthrough reveals how the bacterium can produce a lipid, called 10(Z)-hexadecenoic acid, which directly reduces inflammatory activity in immune cells. The researchers chemically synthesized the compound and discovered how this novel fat molecule directly inhibits key pathways in immune cells that drive inflammation.

"It seems that these bacteria we co-evolved with have a trick up their sleeve," says Lowry. "When they get taken up by immune cells, they release these lipids that bind to this receptor and shut off the inflammatory cascade."

It's an exciting discovery, opening up a number of new pathways for future research. As well as suggesting Mycobacterium vaccae may be an effective probiotic therapeutic in and of itself, the specific fat molecule may be further investigated as a new anti-inflammatory drug. Lowry's ultimate goal is the development of a kind of "stress vaccine" that can be administered to those working in high-stress jobs to help prevent mental health issues from traumatic or stressful scenarios.

Lowry's work doesn't especially verify either the hygiene hypothesis, or its old friends variant, but instead affirms how much more we still have to learn about the effects good bacteria can have on the mechanisms that keep us healthy.

"This is just one strain of one species of one type of bacterium that is found in the soil but there are millions of other strains in soils," says Lowry. "We are just beginning to see the tip of the iceberg in terms of identifying the mechanisms through which they have evolved to keep us healthy. It should inspire awe in all of us."

The new research was published in the journal Psychopharmacology.

Source: CU Boulder