The meme pictured below has been making the rounds on social media and it gets quick acceptance by readers. As gardeners we all know that being in the garden or going for a walk in nature makes us feel good. Finally science agrees with us and has even found the root cause for these feelings of euphoria: serotonin.
People agree with the meme quickly because it supports their exiting beliefs but they have no basis for their existing beliefs.
But let’s face it – a meme is not scientific proof. What does the science really say?
Analyze the Data
When you are confronted with new information like the above meme, take a moment and evaluate it. Do a sniff test to see if it could be valid information.
A quick Google search shows that Mycobacterium vacii does not exist. However, Mycobacterium vaccae does exist. One red flag.
Antidepressant is spelled wrong – second red flag.
How can the smell of bacteria make us feel good? Very suspect to me.
Serotonin and Feeling Good
Serotonin has been well studied and is known as the “feel-good hormone”. It plays a key role in staving off anxiety and depression. So if smell from microbes in soil cause higher levels of serotonin, it is quite likely that these smells make us feel good.
Petrichor is the pleasant smell that frequently accompanies the first rain after a long period of warm, dry weather. Humans are quite sensitive to this smell which is caused by a chemical called geosmin and it has been reported that humans can detect 100 parts in a trillion. We find this smell quite pleasant and geosmin has even been added to perfumes.
Geosmin is made by Streptomyces bacteria. It is not an antibiotic but it does attract insects that then help distribute spores of Streptomyces?
The important point for our discussion is that we now know we can smell compounds made by bacteria in soil. But M. vaccae is not Streptomyces.
Mycobacterium vaccae and Cancer
Mycobacterium vaccae is in the same genus as Mycobacterium tuberculosis, the bacterium which causes tuberculosis. Numerous trials have indicated that exposure to oral and injectable products derived from M. vaccae bacteria can have positive effects in treating tuberculosis. This prompted a doctor named Mary O’Brien to give the bacteria to lung-cancer patients to see if it might improve their condition. What she found was that it made them feel better and have less pain.
Mycobacterium vaccae and Serotonin
Other scientists followed up on this work and slowly deduced the chemistry involved.
When rats were injected with M. vaccae they showed less stress and aggravation following tail shock treatments.
Injection in mice activated serotonergic neurons and caused antidepressant-like behavior.
Dr. C. A. Lowry has done a number of studies to clearly show that M. vaccae does cause an increase in serotonin levels.
Scientific Proof Or Not
So far I have presented some of the science that has lead to the conclusions in the meme and hundreds of other popular articles have made similar claims. It seems like a clear connection. A bacterium in soil causes our bodies to make more serotonin which in turn makes us feed good. Even online magazines like Medical News Today got swept up in the excitement and announced, “friendly bacteria found in soil may affect our brain in a similar way to antidepressants. ” They at least included the all important word “may”, most articles fail to do that.
But here are the problems with this conclusion.
- All testing that showed an increase in serotonin levels has been on rats and mice.
- The bacteria were either injected or fed to the test animals.
I have found no testing on humans that showed a change in serotonin levels after exposure to M. vaccae. I have also not found testing that looks at the effect of “smelling” the air from soil.
It is quite likely that we breathe in some bacteria spores as we work in the garden or walk in the woods, but the amount is probably very small compared to the amounts injected during the studies.
This type of unsubstantiated leap from some scientific facts to a conclusion is very common in news reporting. I think it’s because the facts alone don’t make sensational headlines. This leads to completely inaccurate conclusion like this one:
“Working in the garden, or even taking a walk in the woods can provide enough of a dose of Mycobacterium vaccae to uplift you.”
Or this one:
“Getting your hands dirty in the garden can increase your serotonin levels.”
Neither is supported by science but they do help spawn a new myth.
Shit Might Work Better Than Soil?
The gastrointestinal tract is rich in microbe-generated signaling substances that affect several biological functions in the body (‘inter-kingdom communication’). “The normal smell of human feces is largely due to indole, one of the major metabolites of tryptophan. Tryptophan is an essential amino acid that is used to produce serotonin which has garnered much attention because of its wide physiological and clinical implications.”
The science supporting this meme is at least as strong as the science supporting the meme at the top of this post.