The Good Earth
Australian Geographic Magazine|November December 2019
There’s a growing mound of evidence that spending time in a habitat with healthy soil can be very good for you.
Wilson Da Silva

EVEN ON A BLUSTERY winter’s afternoon, Mount Lofty flaunts its splendour as a bushland oasis, one of the last vestiges of the original forests and woodlands that once dotted the Adelaide Plains. Walking the winding nature trails here, you encounter a multitude of native trees, shrubs, climbing plants, reeds and grasses, two-thirds of which harbour fruit, seeds or insects that attract birds, or nectar that brings butterflies. Meandering down the narrow tracks from the summit, you feel invigorated by the scenery, the silence, the smell of wet earth after a light shower. To a city dweller, the air itself seems therapeutic.

And it’s not an illusion. Every time you enter wild spaces replete with biodiversity and breathe the air, microbes wafting through the ecosystem land on your skin, enter your lungs and gut and become part of you. They join the many billions already living in your microbiome – the community of symbiotic micro-organisms inside each human being.

Scientists have long known about these minuscule, mutually beneficial helpers in our bodies. But thanks to technological developments in molecular imaging, computer speed and rapid genetic sequencing, they have in recent years been astounded to realise how crucial and widespread the role played by our microbiome is in everything from nutrition to disease resistance and even mental health.

The latest surprise is the discovery by researchers in Adelaide that the more diverse the microbes living in the soils around us, and the more you are exposed to them, the healthier you become. It’s a discovery that could change our cities, and make us healthier, on a global scale.

And it all comes down to humble earth.

THE HUMAN BODY is made up of an estimated 37 trillion individual cells, from skin and liver cells to brain and gut cells. But the microbes – micro-organisms such as bacteria, viruses, protozoa and fungi – living in, and on, our bodies are thought to be more than double that, numbering about 100 trillion. While most of us might not give much thought to these tiny interlopers, they are beneficial – even essential – to our survival, many doing things our bodies can’t.

Our microbiome is an enormous, invisible ecosystem that acts like a giant shadow organ, helping us digest plant matter, manufacture vitamins, regulate our immune system, form new blood vessels, coordinate hormone activity, store fat and modulate brain signals. In fact, its role is now considered so crucial that some leading researchers have argued that animals and plants are not autonomous entities but holobionts – dynamic biomolecular networks of different tiny species working in a symbiotic relationship for mutual benefit.

Some 70 per cent of microbes in the human body occur in the gastrointestinal tract – the mouth, oesophagus, stomach and intestines, which together make up the organ system that takes in food, digests it to extract and absorb energy and nutrients, and expels the remainder as waste. Study after study during the past 20 years has identified a marked difference in the gastrointestinal microbes of people living in rural surroundings compared with those living in cities.

Country dwellers have more diverse microbiomes; in city folk the diversity is much less. A lack of microbe diversity is also seen in people who suffer inflammatory diseases, such as asthma and food allergies, which plague city dwellers and the incidence of which has soared in highly urbanised populations.

“There’s a whole host of benefits we derive from living in healthy environments,” says Dr Philip (Phil) Weinstein, a University of Adelaide biological sciences professor with qualifications in both public health and ecology. “One of them is the ecosystem we’re exposed to – the rich diversity of microbes in the soil, in the air, in the food we eat and the animals we interact with.

“For most of our history as a species, we’ve had exposure to that, and that’s what we’ve evolved with and are adapted to live in – as well as eating high-fibre foods and running around all day. But the movement of people into cities has basically cut us off from that environment. And all of those diseases of the urbanised Western lifestyle – like cardiovascular disease, diabetes, obesity, asthma – have increased dramatically.”

Researchers now know there’s a connection. But the focus of health professionals has been on diet and exercise alone. Little attention has been paid to the role ecosystems around us can play in improving our health, particularly gut microbiome health.

Although we can’t reverse urbanisation and return to the savannahs of Africa where humans evolved, we can bring those ecosystem benefits back to our cities. And that’s exactly what Phil and his colleague Dr Martin Breed are trying to do.

The duo run the Healthy Urban Microbiome Initiative (HUMI), which has been exploring how adding microbial diversity to urban environments might improve the health of people by boosting the population of good bacteria in their microbiomes.

“There are more species of bacteria, fungi and other microbes out there than anything else – all this incredible and mostly unknown diversity, which performs all sorts of different functions for not just us but the natural world,” Martin says. “Plants form symbiotic relationships with many different microbial species, which provide the resources they need to grow. In healthy soil in the wild, in a quarter of a gram of soil, you often find 2000–5000 species – which is phenomenal.

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