The soles of your feet can tell us where you call home

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Biology / Environment / Evolution

A step above GPS, the individual microbiomes of a city can be as unique as a fingerprint and more diverse than a rainforest.

Share the love: this post was written by University of Melbourne Science Communication student Morgan Spencer.

Scientists can work out what cities you have spent time in based on the microbiome on the soles of your shoes. Image by Ryoji Iwata on Unsplash.

But I thought the microbiome had something to do with your gut?

Microbiomes are the communities of microorganisms that exist in a particular environment and they may be helpful or harmful to the place they call home. These microorganisms can include any number of bacteria, fungi, viruses and parasites like microscopic worms. Communities of microbes can exist anywhere that meets their requirements for food, moisture and light or warmth. For example, the microbiome of your digestive tract is a specialised microbe community that helps to break down food into the particles your body uses as fuel. Some microbes can only exist in very specific places and as a result, can be hard to find. And it turns out some microbes are so unique to particular places, they can act like landmarks as distinct as the Eiffel Tower or the Sydney Harbour Bridge.

Vacation anyone?

Areas that have lots of people in them like train stations and hospitals have been shown to be a breeding ground for microbes, no matter where you live in the world. Your skin, and by extension, your clothing and shoes become colonised (some might say contaminated) with microbes. At the same time, these microbes adapt and evolve to changes in the environment around them.

Factors like climate and geography play a huge role in what microbes live where. The fact we love to travel enables microbes to move by trains, planes, and automobiles to new, more hospitable locations. Environmental factors also help to determine which microbes hitch a ride with us, meeting the people we encounter on our travels. Recent research shows no two cities share the exact same microbiome.

The leader of this research, Dr. Christopher Mason, sees high transit areas like train stations as significant and undocumented ecosystems of biodiversity. His passion for the world of microbes began when his young daughter naively licked a pole on the street. Sounds gross, especially in the highly sanitised world many of us now live in. But to Mason, this was a fascinating interaction between his daughter and the world around her and set him off on a new research path.

Mason, a microbiologist by trade, spearheaded a local New York project, swabbing subway turnstiles, railings, and benches. Mason and his team started to identify new species of microbes very early on: microbes that had never been observed by scientists previously. It turns out that surfaces inside train stations and airports may hold the ticket to a totally unseen world.

In 2015, a consortium of the International Metagenomics and Metadesign of Subways and Urban Biomes was launched to standardise how researchers collect microbe samples in these places. Since then, samples have been collected in major cities from 60 countries across the Americas, Europe, Asia, Oceania, Sub-Saharan Africa and the Middle East. Researchers use lab techniques to identify both the abundance and diversity of microbes in every sample.

Taking a walk on the wild side

The researchers analysed more than 4,200 different samples and found almost 11,000 new viruses and more than 1000 new bacteria. Perhaps some of these could be the key to new medicines. But this research extends far beyond these new discoveries: it also has the potential to promote the surveillance of these microbiomes for antimicrobial resistance. As medicine has advanced and we’ve come to depend more and more on drugs to treat infections, many epidemiologists, immunologists and doctors are hugely concerned about antimicrobial-resistant microbes. Even closer to home, this network of unique microbiomes could even allow for the surveillance of impending pandemics, allowing our immunologists a head start in public health strategies, vaccine development and prevention.

Environmental microbiomes may be as uniquely identifiable as snowflakes and this global project highlights the benefits of microbial surveillance and mapping to help assess potential health risks and to identify and familiarise ourselves with new species, some of which could be beneficial. The scientists who did this research claim that if you give them your shoe, they’ll be able to tell you with 90% accuracy which city you come from. Who knew a dirty shoe could be so informative?

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