Are there soil microbes under my fingernails?

If you’ve been gardening without gloves lately, then the answer is probably “yes.” But that is, for the most part, a good thing – besides having to clean your hands!

In garden soil there are millions to billions of microorganisms, or microbes. They could be bacteria, fungi, viruses, and archaea. Baseline estimates suggest that there are tens of thousands of different soil microbes.

Soil health and biodiversity are closely linked with human health. Some of the health-relevant roles played by soil have been well-investigated, such as the ability to:

hand holding red potato with soil underneath fingernails
Gardeners and small-scale farmers are more likely to be exposed to soil microbes when digging in the soil. Here, a red potato freshly harvested. Credit: SV Fisk.

Other important functions, however, remain poorly characterized. Little is known about interactions between farmers or gardeners and soil microorganisms. They are both exposed to soil microbes while doing their work.

Gardeners are more likely to cultivate the soil with their hands than most farmers. They are more likely to experience close and extended contact with microorganisms in the soil. However, we currently lack even the most basic understanding of how much microbial transfer from soil to skin occurs, what types of microorganisms are transferred, or how long they persist. That’s the focus of my post-graduate research.

From a microbiological point of view, gardens are quite different from other types of greenspace. Gardens are interactive places where there is often intimate and extended human contact with the soil. In a garden, we dig our hands into the soil, aggressively pulling weeds or tenderly planting a new seedling. We may have soil (and microbes) under our fingernails for an extended period afterwards—hours or even days.

Perhaps we also consume vegetables directly as we harvest them. We might brush off an occasional bug or speck of dirt, but we don’t clean our garden veggies in the same meticulous manner as commercial producers. All of these interactions with soil and garden plants have the potential to influence our own microbiomes.

male gardener collecting soil samples
A Master Gardener collecting soil samples. These will be tested to determine which types, and what quantities of soil microbe are in this location. Credit: Jack Lazareck

But don’t get too concerned – because most of the microbes you might encounter in garden soil are more likely to be neutral, in terms of health effects, rather than causing illness. Furthermore, emerging evidence suggests that exposure to soil microorganisms can help train the immune system. There is also a link to reduced inflammation, and even improved mental health. For example, the common soil bacterium Mycobacterium vaccae has been found to have positive impacts on stress tolerance and mental health.

To determine the full impact soil microbes have on human health requires that we understand the extent to which microbes transfer to human skin. Recent studies have shown that direct contact with soil and its associated microbiota can leave an imprint on the skin microbiome for at least 24 hours. This is even after washing and bathing! Skin contact with soil microbiota can also impact the human oral and gut microbiomes.

My research focuses on the ability of soil microbes to transfer to human skin during gardening activities and whether they can persist there for a long period of time. I am working with Oregon Master Gardeners in a community science effort. We hope to characterize soil microbial communities in small-scale urban farms or gardens.

hand and cotton swab
Method for collecting skin surface microbial swabs. The swabs are placed in bags and then processed to determine which microbes are present on the skin. Credit: Gwynne Mhuireach

First, we gather soil and skin swab samples from participating gardeners. Next, we will use next-generation DNA sequencing to identify microbes present in garden soils across Oregon. We’ll explore whether skin microbiome composition changes due to gardening activities. And we’ll determine how long it takes for the skin microbiome to return to pre-gardening baseline.

I anticipate that this project will demonstrate measurable soil-to-skin microbial transfer immediately after gardening activities. This could be followed by a gradual return of the skin microbiome to the pre-gardening state over a 24-hour period.

Our work will increase our understanding of how many microbes are transferred, what types of microbes they are, and how long they persist. From this, gardeners and farmers will be better able to make important health decisions, such as whether to wear gloves while performing cultivation activities and which hygiene practices are most effective at removing the imprint of the soil microbiome afterward.

Answered by Gwynne Á. Mhuireach, University of Oregon, and Gail Langellotto, Oregon State University.

Interested readers can follow Dr. Langellotto and her team’s work on the Garden Ecology Lab blog:

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