Numerous intriguing claims about gardening have circulated, especially one that insists, “The soil acts as an antidepressant.”

According to this notion, it’s been promoted through countless social media posts. Mycobacterium vaccae, microorganisms commonly found in soil, are said to improve your mood. Simply engaging with the earth can yield these benefits. It’s believed that these bacteria can be absorbed through your skin or inhaled, subsequently enhancing your brain chemistry. But is this as credible as it seems?

While these claims may appear peculiar at first glance, studies have indeed explored the effects of this microorganism on various health conditions, such as eczema and cancer. Interestingly, M. vaccae was first identified in Ugandan soil samples while scientists sought a non-lethal relative of Mycobacterium tuberculosis, and it has potential as a form of immunotherapy.

Researchers became intrigued by its possible benefits for depression when lung cancer patients treated with this bacteria reported improvements in their quality of life, which was an unexpected yet welcomed side effect. Current research, likely indicating an uplift in mood, has been replicated across numerous well-designed studies. Thus, the internet is rife with memes about this finding.

However, there is a caveat. All studies specifically examining this hypothesis have been conducted on mice rather than humans, which is significant because the outcomes of animal studies are often difficult to extrapolate to humans. For instance, one review of 76 animal studies found that only 37% were replicated in human trials.

Moreover, the mice used in the M. vaccae studies were male and from specific inbred strains. Researchers varied their methods for administering the bacteria, either by saturating the air in their cages or applying it directly to their skin. Most studies I found involved injecting the bacteria into the bloodstream of the mice or incorporating it into their food.

As someone captivated by the growing evidence that suggests spending time in green spaces improves mental health, I eagerly anticipate further research on M. vaccae. Nevertheless, despite the viral nature of the claim that “soil is an antidepressant,” it’s essential to acknowledge that it primarily stems from studies on male mice injected with purified bacteria.

James Wong is a botanist and science writer with a particular focus on food crops, conservation, and the environment. He trained at the Royal Botanic Garden in Kew, London, and shares a small flat with over 500 houseplants. Follow him on X and Instagram @BotanyGeek

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