We know that productive garden soil contains between 100 million and 1 billion bacteria per gram (about a teaspoon full). We also know that many of these bacteria have important roles to play in the Soil Food Web and are to a great degree orchestrated by plants through plant root exudates. What we don't know is what roles many of the different bacteria play. We know about a small fraction of them that help with nutrient cycling, disease suppression, and are important to water dynamics in the soil but the greater number by far and their related interaction with other organisms is currently beyond our understanding. In 2007, Christopher Lowry, a neuroscientist now at the University of Colorado found that certain strains of soil-borne bacteria sharply stimulated the human immune system. One bacteria, however, Mycobacterium vaccae, has been found to have amazing benefits to us directly.
Mycobacterium vaccae, a bacteria that is common in soil, has been found to activate a specific group of neurons in our brains that produce serotonin. Serotonin is found in the brain and the blood and is a very important neurotransmitter which helps regulate a whole host of functions including:
- behavioral arousal
- motor activity
- coping responses to stress
Many antidepressants work by serotonin pathways or serotonergic systems. Antigens derived from the bacteria, which is non-pathogenic and saprophytic in nature, altered stress-related emotional behavior in mice. The tests and results of studies in London and Bristol showed that the bacteria caused an increase in serotonin metabolism within the prefrontal cortex. The conclusions from the 2007 studies included "a novel hypothetical framework for investigating the relationships among immune activation, serotonergic systems, and mental health." This added to previous work showing "unexpected improvements in quality of life scores" to clinical cancer and inflammatory disorder trials. Another trial using the soil bacteria to treat psoriasis showed Mycobacterium vaccae "gave long-lasting clinical benefit to most patients, with minimal side effects."
Related to M. vaccae are two scary bacteria: M. leprae and M. tuberculosis which cause leprosy and TB. M. vaccae, named after the cow, however, because it was first isolated from cow manure, is ubiquitous in fertile soil and thrives anywhere soil is enriched with organic matter. Christopher Lowry said that the effect of M. vaccae is "basically no different from all the SSRIs". This is exciting: SSRIs are antidepressants and include medicines such as Prozac, but whereas SSRI's are considered "blunt instruments" by many scientists for treating mood disorders, M. vaccae are selective and specific in their ability to excite certain neurons that produce serotonin and directly impact cognition and mood regulation as well as increase the ability to cope with stress and anxiety.
In a 2008 article in Psychology Today, Daniel Marano explains that eating antioxidant-rich fruits and vegetables may only be half of a newly evolving story of health and that our society's displacement from the natural world has disrupted a deep and direct connection we have to the soil and it's resident organisms. Indeed, the sharp rise in chronic asthma, inflammatory bowel disease, and other inflammatory, allergic, and immune disorders has resulted in "The Hygiene Hypothesis". The bottom line is that we as a society have gotten too clean and the recent obsession with sanitation and anti-microbial products has caught up to us. It's clear that we are a part of the Soil Food Web and removing us from the soil is to our detriment!
Not only does M. vaccae promote the generation of serotonin in our brains, but also norepinephrine.
Perhaps more important, it induces the neurogenesis, or creation, of more neurons that produce these two amazing compounds in our brains. Norepinephrine is just as important as serotonin and affects the amygdala part of the brain where attention responses are controlled, including the "fight-or-flight response and related heart rate, glucose release, and blood pressure responses. Norepinephrine plays a large role in attention and focus and may be used for indications of attention-deficit/hyperactivity disorder, depression, and hypotension.
Similar to the striking rise in allergic and inflammatory disorders, the rise in ADHD and Depression and the related increase in the use (and abuse) of drugs such as the psychostimulants Ritalin or Concerta as well as tricyclic antidepressants or newer SNRI's such as Effexor and Cymbalta point towards a modern society who's young kids do not get to play in the soil enough and one who has abused the use of antibiotics. It would seem that exposing ourselves and especially our children to the soil in the garden could be a good thing indeed. The Hygiene Hypothesis page in Wikipedia now has 39 references as well as plenty of supporting articles for further reading and external links. It "has become an important theoretical framework for the study of allergic disorders" and both their increase since industrialization as well as their higher incidence in more developed countries lends credence to the theory which was first given a scientific background by David Strachan in 1989 and later made popular in the 2005 book by Richard Louv called "Last Child in the Woods: Saving Our Children from Nature-Deficit Disorder." This is now a movement called No Child Left Inside and a Federal bill introduced to the House of Representatives on April 22, 2009, Earth Day. The bill is currently languishing in a subcommittee on early childhood education due to opposition from a conservative american "think tank" called The Heritage Foundation.
It is now clear that exposure to bacteria, and especially during neo-natal development helps to adequately develop regulatory T cells. In 2007, Keven Becker of the Gene Expression and Genomics Unit at the National Institute on Aging wrote an article that supports a link to Autism, also on the rise, to the hygiene hypothesis and shows etiological links between autism, asthma, and inflammation disorders. He hypothesizes that "immune pathways altered by hygiene practices in western society may effect brain structure or function contributing to the development of autism."
More recently, Dorothy Mathews and Susan Jenks of the Sage Colleges in New York tested live mice by feeding them live M. vaccae and found that they could "navigate a maze twice as fast and act with less demonstrated anxiety behaviors as control mice". Mathews said in a May 25 2010 Science Daily article that "learning environments in schools that include time in the outdoors where M. vaccae is present may decrease anxiety and improve the ability to learn new tasks." This is a pretty good reason to promote school gardens!
So what then, do all these medical maladies have to do with gardening, horticulture, and landscaping?... the point here is that we as organisms co-evolved on this planet along with and as a part of the soil ecology over millions of years. Over the past 100 years or so we have become increasingly detached from the soil that supports us and our efforts to further cut us off from bacteria and microbes by the use of antibiotics and antibacterial products has led to increases in diseases and disorders. It's time we realize the health benefits of our natural connection to the Web of life and rejoice in the practice of getting dirty. The next time you breath in the steamy aroma of fresh turned earth, compost, and manure, think of all the wonderful biology making it's way into your personal ecosystem. Contemplate the increased biodiversity you now harbor and be glad. While there are still valid concerns over other bacteria such as certain strains of Escherichia coli, for the most part, the little soil that sticks on the fresh veggies you eat may actually be covered in organisms that are very good for you. -Yet another great reason to grow your own food and if you don't have one, start a garden today!