Are there soil microbes under my fingernails?

April 15, 2021 – If you’ve ever touched soil with your bare hands, you’ve come in contact with soil microbes. Gwynne Á. Mhuireach of University of Oregon and Gail Langellotto of Oregon State University, discuss the potential health benefits and risks of direct soil microbe exposure in this Soils Matter Blog:

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:

• produce nutritious food,
• hold carbon,
• lessen the effects of environmental pollutants, and
• purify water.

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.

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.

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.

For more information about soils, follow SSSA on Facebook at https://www.facebook.com/SSSA.soils, Twitter at SSSA_Soils. SSSA has soils information on www.soils.org/discover-soils, for teachers at www.soils4teachers.org, and for students through 12^th grade, www.soils4kids.org.

The Soil Science Society of America (SSSA) is a progressive international scientific society that fosters the transfer of knowledge and practices to sustain global soils. Based in Madison, WI, and founded in 1936, SSSA is the professional home for 6,000+ members and 1,000+ certified professionals dedicated to advancing the field of soil science. The Society provides information about soils in relation to crop production, environmental quality, ecosystem sustainability, bioremediation, waste management, recycling, and wise land use.