The Microbiome-Nature Connection: Dirt, Plants, and Immune Health

There's a hypothesis in immunology that sounds like it belongs in a fairy tale: that humans need to be exposed to the ancient microbial companions they evolved alongside — and that modern life, in stripping away that exposure, is quietly making us sick.

The Old Friends Hypothesis, proposed by immunologist Graham Rook in the early 2000s, argues that the human immune system evolved in constant contact with a specific set of organisms — soil bacteria, intestinal parasites, environmental microbes — over millions of years. These "old friends" helped calibrate the immune system's regulatory machinery, teaching it what to tolerate and what to attack. Remove them from the equation, and the immune system misfires: attacking harmless substances (allergies), attacking the body itself (autoimmune disease), staying chronically activated even in the absence of real threats (chronic inflammation).

The epidemiological fingerprint of this hypothesis is everywhere. Allergies and autoimmune diseases are rising sharply in industrialised countries while remaining far rarer in low-income countries and traditional rural communities. The same diseases are more common in urban than rural populations. Children raised on farms — in contact with livestock, soil, and diverse microbial environments — have dramatically lower rates of asthma, hay fever, and multiple sclerosis than urban children with equivalent genetic backgrounds.

None of this is an argument for poor hygiene or avoiding antibiotics when medically necessary. It's an argument that deliberate, positive exposure to natural microbial environments — gardening, time in soil, contact with animals, time outdoors in natural settings — may be as important to health as the foods we eat and the exercise we take.

Soil is not dirt. It's one of the most complex ecosystems on earth — a cubic centimetre of healthy garden soil contains approximately one billion bacteria, representing thousands of species, alongside fungi, nematodes, protozoa, and a vast network of mycorrhizal connections.

When humans garden — digging, planting, handling soil, working with compost — they expose their skin, respiratory tract, and gut to this diversity in ways that have been part of human experience for the entire span of agricultural civilisation, and long before it.

What gardening does to the microbiome Research comparing the skin and gut microbiomes of people who garden regularly with those who don't finds measurable differences in microbial diversity. Microbial diversity in the human gut is associated with better immune regulation, lower rates of allergic and autoimmune disease, and better mental health outcomes. The directionality — whether gardening causes better microbiome diversity or whether healthier people are more likely to garden — is difficult to fully disentangle, but the biological plausibility of the mechanism is strong.

The community garden finding A Finnish study exposed children to either standard sand in a playground or soil-and-plant material from a diverse outdoor environment. After 28 days, the children with the diverse outdoor environment showed significant changes in their skin and gut microbiome composition — and measurable upregulation of immune regulatory markers. The effect appeared within a month of regular contact.

What this means practically Gardening, barefoot walking on grass, playing in soil, handling animals — these aren't just pleasant activities. They're microbial exposure events with measurable physiological effects on immune regulation. The degree to which a manicured, sanitised park environment delivers these benefits is lower than an actively worked garden or a more naturalistic setting.

In 2007, neuroscientist Christopher Lowry stumbled onto something in a mouse study that he wasn't looking for. He was investigating a soil bacterium — Mycobacterium vaccae — that had been given to cancer patients as an experimental immunotherapy. The patients reported improved mood and quality of life beyond what the oncological outcomes explained.

Lowry injected M. vaccae into mice and observed their behaviour. The mice ran mazes faster. They behaved less anxiously. Analysis of their brain tissue found elevated serotonin levels — the neurotransmitter targeted by antidepressant medications — in the prefrontal cortex.

The mechanism appeared to be immune-mediated: M. vaccae activated specific regulatory T cells that suppressed inflammation and triggered serotonin production via a pathway involving tryptophan metabolism. The researchers hypothesised that exposure to soil bacteria — a ubiquitous feature of human environments throughout evolutionary history — was one of the signals that kept the human stress and mood regulatory systems in healthy calibration.

The implications This research is preliminary and hasn't translated into a licensed therapy. But it provides a compelling mechanistic link between soil exposure and mood regulation — and it helps explain why gardening is consistently associated with lower rates of depression and anxiety in epidemiological research, independent of the exercise and social components that gardening involves.

Immunological priming, not treatment M. vaccae's effects appear to operate through chronic low-level immune priming — the kind that comes from regular, recurring exposure to natural environments — rather than from a single large dose. Gardening, regular outdoor time in natural settings, and contact with diverse natural surfaces are the exposure patterns the research points toward.

You don't need a farm or a countryside to increase your microbial nature exposure meaningfully. What you need is deliberate, regular contact with natural living systems.

Gardening Even a small container garden or window box involves soil contact that delivers microbial exposure. Community garden plots — available in most cities — provide access to more diverse soil environments. The key is actual hands-in-soil contact, not just proximity.

Barefoot contact with natural surfaces Walking barefoot on grass, soil, or sand is one of the most direct forms of skin microbiome exposure. The soles of the feet have significant absorptive capacity. Even 15–20 minutes of barefoot outdoor contact several times per week is a meaningful change from a life conducted entirely in shoes on sealed surfaces.

Children and soil play The developmental window for immune calibration appears to be most critical in early childhood — the first five years. Children who play in natural soil environments, interact with pets and farm animals, and spend time in diverse outdoor settings during this window have measurably better immune regulation outcomes than those raised in sanitised indoor environments. The specific instruction to "not let kids get dirty" contradicts what the developmental immunology research now suggests.

Natural swimming and water environments Wild swimming — in lakes, rivers, and the sea — exposes the skin and mucous membranes to diverse aquatic microbiomes that are entirely absent from chlorinated pools. Multiple studies find associations between wild swimming and improved skin microbiome diversity, though research here is earlier-stage than the soil microbiome literature.

Log your nature exposure in PollutionProfile's Nature Exposure tracker and notice whether your entries include active physical contact with natural environments — not just walking through them.