Humans’ modern-day obsession with keeping their homes and bodies regularly disinfected and as close to sterile as possible has come at a steep price — the lack of regular communing with beneficial microbes in the environment.
The issue is only compounded by the fact that many people no longer spend much time outdoors working, playing or otherwise cohabitating with the soil and all of its biological inhabitants.
Within the soil, it turns out, are a host of useful microbes, of which researchers have only uncovered the tip of the iceberg, but what’s been revealed so far is fascinating. University of Colorado Boulder researchers recently found, for instance, bacteria in the soil that’s a source of anti-inflammatory fat — exposure to which may benefit mental health.1
Bacteria in soil may contain stress-relieving fats
Mycobacterium vaccae (M. vaccae) is a type of bacteria found in the natural environment, including soil. The bacteria have previously been found to have anti-inflammatory, immunoregulatory and stress resilience properties,2 as well as offer anti-anxiety effects,3 but the reasons why remained unclear. Now, research published in the journal Psychopharmacology may have unveiled a key piece of the puzzle.4
When evaluating M. vaccae, researchers identified and isolated 10(Z)-hexadecenoic acid, a novel fatty acid that they then sequenced to determine how it interacted with immune cells.
The fatty acid, or lipid, proved to bind to peroxisome proliferator-activated receptor (PPAR), inhibiting inflammation-boosting pathways. Further, when they treated cells with 10(Z)-hexadecenoic acid, they were better able to resist inflammation.5
Christopher Lowry, the study’s senior author, said in a news release, “We think there is a special sauce driving the protective effects in this bacterium, and this fat is one of the main ingredients in that special sauce.” He added:6
“It seems that these bacteria we co-evolved with have a trick up their sleeve,” said Lowry. “When they get taken up by immune cells, they release these lipids that bind to this receptor and shut off the inflammatory cascade …
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. 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.”
Why gardening is an excellent form of stress relief
Gardening presents a trifecta of stress-reducing potential, including physical activity, exposure to beneficial sunlight and, last but not least, exposure to microbes. M. vaccae, for instance, has been shown to reduce anxiety-related behavior and improve learning in mice,7 and it’s possible people inhale such mood-boosting microbes when they’re outside playing or working in the dirt.
In 2004, researchers added M. vaccae to standard chemotherapy for cancer patients, which resulted in significantly improved quality of life.8 In 2007, Lowry and colleagues revealed that M. vaccae activated serotonin-producing neurons in mouse brains — specifically those involved in the immune response.9,10
In 2016, Lowry and colleagues published another study on the remarkable bacteria, showing that when mice were injected with a heat-killed preparation of M. vaccae, then housed with larger, aggressive males, they showed less anxiety and fear-like behaviors.11 The mice also had a 50 percent lower risk of suffering from stress-induced colitis and had less systemic inflammation.12
The results suggest, according to the researchers, that M. vaccae may help prevent disorders similar to post-traumatic stress disorder (PTSD) in mice:13
“Treatment of mice with a heat-killed preparation of an immunoregulatory environmental microorganism, Mycobacterium vaccae, prevents stress-induced pathology. These data support a strategy of “reintroducing” humans to their old friends to promote optimal health and wellness.”
Likewise, Lowry stated that as humans have moved into urban areas, away from their agricultural or hunter-gatherer roots, they’ve lost contact with beneficial organisms that helped to regulated their immune systems and suppress inflammation. “That has put us at higher risk for inflammatory disease and stress-related psychiatric disorders,” he said.14
Soil may also be a source of powerful antibiotics
Antibiotic-resistant disease is spreading across the globe. Every year at least 2 million Americans acquire drug-resistant infections and 23,000 die as a result. Many others die from conditions that were complicated by antibiotic-resistant infections.15
One study estimated that up to 50% of pathogens that cause surgical site infections, and 25% of those that cause infections following chemotherapy, are already resistant to common antibiotics.16 Unless steps are taken to curb antibiotics usage (including in agriculture), it’s possible we could return to an era where antibiotics no longer cure common bacterial infections — but it’s possible soil could come to the rescue.
Researchers at the Rockefeller University in New York City analyzed about 2,000 soil samples from the U.S., extracting DNA and screening it to turn up a new family of antibiotics called malacidins.17 The compounds were powerful enough to kill multidrug-resistant bacteria in laboratory tests.18
In 2018, researchers tested soil samples from Northern Ireland, also revealing a potential source of antibiotics. They were looking for the presence of Streptomyces, which are known to produce antibiotics, and a novel strain they named Streptomyces sp. myrophorea.19 The new bacterial strain was also capable of inhibiting the growth of multidrug-resistant pathogens.20
Interestingly, there are historical accounts of people using soil, such as the red soils from the Hashemite Kingdom of Jordan, to treat infections. When researchers collected samples of soil from the area and inoculated them with bacteria, the bacteria were quickly killed, and a number of antibiotic-producing bacteria were identified in the samples.21,22
One of the authors of the Ireland study, professor Paul Dyson of Swansea University Medical School, explained in a news release that looking toward traditional medicines, like soil, could be the answer to modern-day problems:23
“This new strain of bacteria [Streptomyces sp. Myrophorea] is effective against 4 of the top 6 pathogens that are resistant to antibiotics, including MRSA. Our discovery is an important step forward in the fight against antibiotic resistance.
Our results show that folklore and traditional medicines are worth investigating in the search for new antibiotics. Scientists, historians and archaeologists can all have something to contribute to this task. It seems that part of the answer to this very modern problem might lie in the wisdom of the past.”
Trees also release stress-relieving compounds
In Japan, a practice known as Shinrin-yoku, or forest bathing, is a popular pasttime. In a study that compared the health effects of spending time in a forest versus spending time in a city, the forest environment was found to promote lower concentrations of cortisol, lower pulse rate, lower blood pressure, greater parasympathetic nerve activity and lower sympathetic nerve activity.24
Forest bathing has also been shown to offer relaxation effects while decreasing symptoms of depression, fatigue, anxiety and confusion in middle-aged men.25
It’s even been found that visiting a forest increases the activity of natural killer cells, a part of the immune system, as well as the expression of anticancer proteins — beneficial effects that persisted for at least seven days after the visit to the forest.26 Volatile compounds called phytoncides, such as such as alpha-pinene and beta-pinene, are released from trees and found in forest air.27
It’s thought that phytoncides released from trees, as well as reductions in stress hormone, may be partly responsible for the increased activity of killer cells.28 When 498 volunteers spent time in a forest, they experienced significant reductions in stress levels, including lower scores in feelings of hostility and depression. The researchers wrote in the journal Public Health:29
“This study revealed that forest environments are advantageous with respect to acute emotions, especially among those experiencing chronic stress.
Accordingly, shinrin-yoku may be employed as a stress reduction method, and forest environments can be viewed as therapeutic landscapes. Therefore, customary shinrin-yoku may help to decrease the risk of psychosocial stress-related diseases, and evaluation of the long-term effects of shinrin-yoku is warranted.”
The case for eating dirt
Many have taken on the attitude that getting dirty is a bad thing, but when that dirt comes from a natural (nonpolluted) environment, the opposite is actually true. In many parts of the world, in fact, even eating dirt is common, especially among pregnant women.
Writing in Emerging Infectious Diseases, Gerald Callahan, a professor in the department of microbiology, immunology and pathology at Colorado State University, suggests this may be done to boost the mother’s immune system, helping her to produce high levels of antibodies against pathogens in the environment, which would then appear in breast milk and offer further protection for the infant.30
“Eating dirt, then, rather than being abnormal, may be an evolutionary adaptation acquired over millennia of productive and not-so-productive interactions with bacteria — an adaptation that enhances fetal immunity and increases calcium, eliminates gastric upset, detoxifies some plant and animal toxins, and perhaps boosts mothers’ immunity,” he notes.31
He also points out the common habit that children have of eating dirt. It’s so common that the U.S. Environmental Protection Agency estimates children consume between 200 milligrams and 800 milligrams of dirt daily.32 While parents may try to stop this, Callahan notes, this exposure to dirt and the microbes therein is likely an integral part of healthy development:
“We parents have tried for years to put a stop to it. I don’t know of an instance in which anybody has succeeded in keeping children away from dirt. But animals have been successfully raised in absolutely sterile environments.
Rabbits, mice, guinea pigs, and rats have been raised under such conditions. In each case, the immune system failed to develop normally … Evidence suggests that the results would be the same in children.”
While, unfortunately, many areas have soil polluted with heavy metals, agricultural chemicals and other toxins that would make eating it dangerous, the underlying premise is the same: Don’t be afraid of getting dirty. Spend time outdoors, work in your garden and enjoy a tomato or two picked fresh from the vine.
Soil-based spores, or sporebiotics, are another option you can use to help introduce more beneficial microbes to your body, but you can also make a conscious effort to get out in nature as much as possible, put your hands and feet in the dirt, and re-establish the natural connection that’s been an innate and beneficial part of being human for millennia.