Cleanliness can stand alongside godliness. But it turns out being a deity is pretty unnerving.
Growing up in carefully sanitized conditions, devoid of the “old friends” germs and parasites that co-evolved with us and help train our immune system, makes us more susceptible to a host of health problems. These include inflammatory bowel disease, allergies, asthma and autoimmune diseases, scientists have discovered. The prevalence of these health problems has skyrocketed in the squeaky clean developed world. But that is not everything. According to a growing body of research, this well-established hypothesis, the hygiene hypothesis, may also explain the rise in certain mental health problems.
The same inflammation and confused immune responses that can be explained by the hygiene hypothesis have also been linked to depression, anxiety, and stress disorders, such as post-traumatic stress disorder (PTSD). Studies have shown that high levels of inflammation increase the risk of developing depression, for example, and PTSD is associated with pro-inflammatory signals and decreased regulatory T cells — cells that suppress immune responses, including inflammation.
Immune signals — including errant ones — can alter brain signals, the brain’s release of hormones and the neural circuits for mood, researchers have found. While the findings underscore the importance of our microbial communities to our health, they also offer tips on how to treat such mental health issues.
Step aside, probiotic yogurt
With a new appreciation for the microbiome and its role, researchers are eagerly chasing ideas to harness the power of microbes and improve health, particularly by attempting to reconcile with bygone microbes. To that end, entrepreneurs have developed probiotic foods and bacteria-laden body washes — with questionable success. But now scientists are tackling the mental health connections and have recorded some promising early results.
By injecting mice with dead batches of a once-common gut microbe, researchers show they can manipulate rodents’ immune systems, alter gene regulation in their brains and reduce stressed and anxious behaviors. While the study, published in the Procedures of the National Academy of Scienceswas only performed in a small group of mice, the findings point to future therapies that could appeal to members of the microbial community to block psychiatric disorders.
“These data support a strategy of ‘reintroducing’ people to their old friends to promote optimal health and wellness,” the authors conclude.
For the study, the researchers chose heat-killed Mycobacterium vaccaean abundant soil dweller that feasts on dead or decaying organic matter and is known to stimulate immune responses in mice.
Compared to control mice that received no vaccine from M. vaccae, the injected mice showed less anxious and submissive behavior in a stress test. The test involved throwing males into a cramped cage with a dominant male and monitoring their behavior. While 95.7 percent of the control mice showed at least one submissive behavior in the first hour, such as standing up, avoidance and flight behaviors, only 65.3 percent of the vaccinated mice did. In addition, the vaccinated mice scored higher on dominance indices, which accounts for chasing and fighting behavior.
When the researchers glimpsed the mice’s brains, they found that the vaccinated rodents had turned on genes involved in coping strategies. And while the control mice generally developed stress-induced colitis — inflammation of the colon — during the month-long test, the vaccinated mice did not.
Finally, when the researchers gave the vaccinated mice an antibody that knocks down the activity of regulatory T cells — those anti-inflammatory cells — the vaccine’s protective effects were wiped out.
For now, it’s too early to extrapolate the specific findings to human health. But the study suggests that reintroducing humans to the microbes of their dirty days of yore could improve mental health. “Restoring exposure to these old friends through immunization or other routes may reduce vulnerability to inflammation-related disease in modern urban societies,” the authors write.
Procedures of the National Academy of Sciences2016. DOI: 10.1073/pnas.1600324113 (About DOIs).