We know that eating a variety of organic fruits and vegetables promotes good health…

We also know spending time in nature, whether it’s walking through green spaces or spending time in the garden, can increase feelings of well-being and reduce cortisol, the stress hormone.

Now researchers have discovered another reason why spending time outside makes us feel so much better: Soil contains a beneficial strain of bacteria that can improve cognition and lower some risk factors for dementia and Alzheimer’s disease

Read on to learn more about this friendly microbe and how it can keep your brain strong…

The helpful bacterium is known as Mycobacterium vaccae (or M. vaccae). It’s abundant in soil. When we’re outdoors we breathe it in the open air…

As you may know, science is homing in on the connection between our microbiome – the beneficial microbes in our gut — and brain function. Some researchers even go so far as to call the gut the second brain. (For more information on gut and brain health, see Issue #205.)

A 2013 study published in the journal Behavioral Processes provided more evidence for this theory when researchers Dorothy Matthews and Susan Jenks fed a group of mice live M. vaccae bacteria and then had them run a maze. A control group ran the maze without the benefit of the microbe.

The results were stunning. Mice that ate the bacteria before and during the trials “completed the maze twice as fast as controls and with reduced anxiety-related behaviors.”1

M. Vaccae Acts Like an “Old Friend”

The “old friends” or “hygiene” hypothesis of stress-related diseases states that, because we spend less time in nature and overuse antibacterial soaps and other germ-killing aids, our bodies no longer reap the benefits of microbes like M. vaccae, which have helped humans survive and thrive for thousands of years.

The hypothesis gets its name from the “strategy of ‘reintroducing’ humans to their old friends [beneficial bacteria] to promote optimal health and wellness.”2

Without them our bodies fall prey to the negative effects of stress and chronic inflammation, and this adversely affects our bodies’ own ability to prevent diseases.

Chronic inflammation also contributes to depression, which is not only a danger in its own right, but is also a risk factor for Alzheimer’s disease and dementia (see Issue #117).3

In a 2016 study published in the journal Trends in Immunology, researchers “immunized” mice with M. vaccae and found that their exposure to the microbe (or infection, if you will) prevented stress-induced colitis and reduced fear, anxiety, symptoms of inflammation and poor stress management.

The Connection Between M. Vaccae, Serotonin and Alzheimer’s Disease

The reason for the animals’ decreased anxiety in the above studies is that ingestion of M. vaccae stimulates the release of serotonin in the brain. Higher levels of this neurotransmitter elevate mood and decrease anxiety.

Serotonin is the “feel good” neurotransmitter that’s also essential to synaptic functioning. Your neurons need serotonin to communicate with each other. This molecule also plays a crucial role in memory and learning.

Serotonin production naturally slows as we age. Research suggests that this decrease could be linked to depression, decreased cognition, memory problems and Alzheimer’s disease later in life.4

Studies of autopsied brains of people with Alzheimer’s disease have shown serious serotonin deficiencies. Being low on serotonin doesn’t necessarily cause cognitive impairment and Alzheimer’s disease, but it is an indicator that overall brain health and functioning has been compromised.5

Big Pharma has been trying to create drugs that boost serotonin, but so far they haven’t been effective.6 So your best bet to prevent low levels of serotonin is to actively stimulate its production with M. vaccae – and by other means.7

How to Increase Your Intake of M. vaccae

Spending time in green places, city parks and uninhabited woods alike, increases your exposure to M. vaccae. Get outside and breathe the fresh air as often as you can.

Eating fruits and vegetables directly from the tree, shrub or vine also helps you to ingest beneficial quantities of this microbe. As long as they’re grown without pesticides and herbicides, you’ll be pretty safe to eat them without washing. (I’d be wary of vegetables grown in or near the ground, especially in manure-rich soil. Those I’d wash.)

Dig a little deeper and you’ll discover that gardening or otherwise closely interacting with the earth gives you even more M. vaccae.

Long-time gardeners swear that spending time among their plants is therapeutic. Now we have a new reason to think so.

Spend some time playing in the dirt with your kids or grandkids. Try planting a small flower garden that you all can tend together. Grow one or two vegetable plants and eat the produce right off the plant. Or visit a local farm and sample their wares.

As long as you’re outside, eating organic fruits and veggies and getting dirty once in a while, you’ll be doing your brain a favor.


  1. Ingestion of Mycobacterium vaccae decreases anxiety-related behavior and improves learning in mice.
  2. The hygiene hypothesis and psychiatric disorders.
  3. Immunization with a heat-killed preparation of the environmental bacterium Mycobacterium vaccae promotes stress resilience in mice.
  4. Serotonin in aging, late-life depression, and Alzheimer’s disease: The emerging role of functional imaging.
  5. Serotonin deficiency: A new target for Mild Cognitive Impairment and Alzheimer’s.
  6. Role of serotonin in Alzheimer’s disease: A new therapeutic target?
  7. How to increase serotonin in the human brain without drugs.

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