Germs, microbes are good for us

By Rob Dunn

Published: Sat 18 Jul 2020, 11:02 AM

Last updated: Sat 18 Jul 2020, 1:08 PM


We may feel isolated now, in our homes, or apart in parks, or behind plexiglass shields in stores. But we are never alone. I've spent much of the last 20 years studying the many species with which we live: thousands of them, perhaps hundreds of thousands, including fungi, bacteria on our skin and in our guts, and animals ranging from the several species of Demodex mites that live in our pores to the spiders that ride with us from home to home.
In ordinary times, no person is an island. We are connected to other people through touch and words but also through the exchange of species, most benign, some even beneficial-on our bodies, in our homes, and more generally in our daily lives. These species may be bacteria, fungi, protists, and even small animals. You kiss a loved one and transfer life from your lips to their cheek, a shimmer of species.
But now we are aware that the kiss can be dangerous or even deadly. As we isolate ourselves in order to reduce the connections in the web, what happens to the whole society of viruses, bacteria, and mites that exists on and between us? What happens when each person, or at least each home, becomes an island?
This is something ecologists and evolutionary biologists have studied for several hundred years now. On islands, with enough time, some species become more common, some go extinct, and some evolve. 
First, there are species that become rarer. We know from thousands of studies of fragments of forest that, as forests are cut into smaller and smaller pieces, species go extinct. For species that live on bodies, it seems likely that the fewer people who live in your home, the more likely it is for any particular body-loving species to go extinct. If it goes extinct on you, it has fewer places from which to recolonise. In normal times, species pass from one person to another, one being to another, when we touch. The more you bump, the more you share. But in our isolation, we bump and share with fewer people and so colonisation is less likely and extinction more permanent. Indeed, this is what we hope happens with the virus that causes Covid-19: that by disconnecting from one another, we give it no island close enough to land upon.
In forest fragments, losses occur in a predictable order: Predators go extinct first, when there are too few prey. Indoors, leopard mites that eat dust mites that eat our skin as it falls from us everywhere we go are almost certainly more likely to go extinct before the dust mites themselves. So too skin or gut microbes that depend on other skin or gut microbes, the wolves of our bodily Yellowstone.
Species evolve more rapidly, as we know from studies of islands, if they have large populations and multiply rapidly. And if these populations become isolated and face different conditions, they tend to diverge. By studying the microbiome, we can see evidence of previous separations among humans. Lice species diverged genetically among populations of Paleolithic humans as they spread around the world. 
We are not only 'gardening' our microbes by subtracting from their web, absentmindedly weeding; we are also giving them additional new foods with our new quarantine regimes and hobbies, and lack thereof.
The curious reciprocity between the microbial world of our foods and the microbial world of bodies also shows up in yogurt, whose bacteria are originally from human mouths and the guts of mammals. In commercial sourdough bread, the most commonly used bacteria appears to have come from the gut of a rat. Many fermented drinks around the world, such as chicha in the Amazon, rely on human body microbes for fermentation. These fermentations influence our bodies, changing our microbiomes, affecting what we can digest and how we smell. We forget that we, too, are gardens.
Actual outdoor gardens also have the potential to change the species on our skin. We know from studies in Finland that children whose outdoor environments include a greater variety of plants tend to have more kinds and different kinds of bacteria on their skin, including bacteria that help to keep them healthy. Exposing yourself to the wild microbes of the garden and forest can have a big impact on your body's wildlife, though we don't know how much exposure it takes to make a difference. One sample of the skin of a child who grew up in the Amazon rainforest, living a hunter-gatherer lifestyle, found more kinds of skin microbes on the forearm of that child than the total number we observed in a study we did of the belly button microbes of hundreds of Americans. How much would you need to garden to achieve such an effect? I suppose the answer is a lot.
I'm ever more aware of the thousands of species on my own body, in my own house and yard-virtually none of which have been studied, and many of which, though we spend so very much time with them, do not even yet have names.
Rob Dunn is an author and Professor in the Department of Applied Ecology at North Carolina State University. -Zocalo Public Square 
 




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