Urban Soil Health and Human-Microbe Symbiosis

By: Ari Del Tufo

Dirt, the reason children are sent to wash their hands and the reason public buildings have hand sanitizer at entrances. Dirt is unclean, unhealthy, and teeming with bacteria. But what of the theory that human beings need bacteria, need dirt?

In the early 2000s, English microbiologist Graham Rook began developing the “old friends” hypothesis. Rook argued that humans and bacteria co-evolved over the past billion years, developing “old friendships” which are integral to the well-being of each. This is in direct contrast to the common idea that a more hygienic individual (or community) is a healthier one (Bloomfield et al. 2016).

In 2004, medical researchers introduced cancer patients to M. Vaccae, a common soil bacterium, with the hope that this would improve outcomes and decrease mortality. While the bacteria had no effect in this regard, it did have an unexpected positive result: patients dosed with M. Vaccae showed a statistically significant increase in their quality of life. These improvements included a reduction of negative symptoms associated with chemotherapy, lessened pain, better functionality, and improved mood. (O’Brien et al. 2004).

Researchers suggested that M. Vaccae increased quality of life by prompting increased serotonin production in patients, in addition to other hormones and neurotransmitters. A 2007 study concluded that M. Vaccae plays an important role in regulating the brain’s production of serotonin and that increased contact with M. Vaccae can significantly improve an individual’s mood and brain function (Lowry et al. 2007). A deficit of serotonin is linked to chronic depression, bipolar disorder, and many other mental illnesses. (Jenkins et al. 2016; Mahmood et al. 2001)

Despite the common notion that dirt is detrimental to health, research suggests that the microbes in dirt have historically played beneficial roles in regulation our immune systems and brain chemistry. Medically, green spaces and access to nature are generally discussed in terms of sunlight and exercise, but, in addition, these natural microbial interactions are “a new ecosystem service which has been completely ignored in the past –  one of the most important things the ecosystem around us does for us is provide these inputs of microbes that we need” (Rook, 2016).

In recent years, we have created increasingly sterile indoor environments in the name good health, when it is very possible that what we really need is more time with our hands in the dirt.


Bloomfield, Sally F, et al. “Time to Abandon the Hygiene Hypothesis: New Perspectives on Allergic Disease, the Human Microbiome, Infectious Disease Prevention and the Role of Targeted Hygiene.” Perspectives in Public Health, vol. 136, no. 4, 2016, pp. 213–224, doi:10.1177/1757913916650225.

Jenkins, Trisha, et al. “Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis.” Nutrients, vol. 8, no. 1, 2016, p. 56., doi:10.3390/nu8010056.

Lowry, C A et al. “Identification of an Immune-responsive Mesolimbocortical Serotonergic System: Potential Role in Regulation of Emotional Behavior” Neuroscience, vol. 146, 2007.

Mahmood, Tariq, and Trevor Silverstone. “Serotonin and Bipolar Disorder.” Journal of Affective Disorders, vol. 66, no. 1, 2001, pp. 1–11., doi:10.1016/s0165-0327(00)00226-3.

O’Brien, M E, et al. “SRL172 (Killed Mycobacterium Vaccae) in Addition to Standard Chemotherapy Improves Quality of Life without Affecting Survival, in Patients with Advanced Non-Small-Cell Lung Cancer: Phase III Results.” Annals of Oncology, vol. 15, no. 6, 2004, pp. 906–914., doi:10.1093/annonc/mdh220.

Rook, Graham. “Can urban greenspace & biodiversity reverse increases in chronic inflammatory disorders.” Youtube, 2013, https://www.youtube.com/watch?v=dM7cqER3t9U

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