As I type these words at my desk, I’m seeding my house with bacteria. I touch the desk, the light switch, the coffee mug, and the microbes on my hands now coat these objects. I absent-mindedly swish my foot across the floor, and I leave microbes there too. I scratch my head, ejecting a cloud of microbes into the air. You do this too. We all do. We constantly sign the places around us with the microscopic parts of ourselves.
Our bodies are home to trillions of microbes, which outnumber our own cells by at least a factor of three. They and the genes they contain are collectively known as the microbiome, and they influence our lives, health, minds, and more. But the microbiome isn’t confined to our skins. It extends into the world around us.
“You have this continual outreach from your body into the environment,” says Jack Gilbert from the University of Chicago. “You are bleeding microbes into space. It’s the first thing anyone will interact with you on.”
Gilbert has been studying this extended self through the Home Microbiome Project—an initiative to map the microbial character of our homes. The first results are out today, and they show just how quickly our microbes colonise the spaces around us. They also show that these traces can be used as forensic tools, to show whether we stayed in a certain place and how long ago we left.
“This work wonderfully contributes to the growing body evidence that we are intimately connected to our homes in ways that we can’t see – through our microbes,” says Jessica Green from the University of Oregon. “The next step is to understand how to design and care for the places where we live, work and play to create healthy microbial systems.”
The team, including Simon Lax and Daniel Smith, recruited seven ethnically diverse American families via Twitter and other social networks. They aimed for variety. There was a young couple; a young couple with a lodger; someone who lived alone; a family of three; a family of four (that was Gilbert) and so on. Some had pets, others didn’t.
The team trained members of these households to collect microbes from their homes every day for six weeks. They swabbed the kitchen and bedroom floors, light switches, doorknobs, and their own hands, feet, and noses. The team sent each family a sampling kit, diaries, and a chest freezer to store their samples. “People are nervous about getting the public involved in science but this study showed that everyone was good at following instructions,” says Gilbert.
When the researchers analysed the data, it rapidly became clear that each home has a distinctive microbiome, which comes largely from the people who live in it. Light switches and doorknobs look like hands. Floor looks like feet. Kitchen countertops look like skin. We turn out homes into microbial reflections of ourselves.
This happens quickly. As soon as we move into a space, we inject microbes into it, and those bugs colonise the area within 24 hours. One of the young couples demonstrated this in the starkest way: at the start of the study, they were staying in a hotel. After they moved, their new home was microbially indistinguishable from the hotel room. “People always say, “Ewwwww, someone else was in this room and it has their microbes all over it.” That’s irrelevant,” says Gilbert. You are constantly overwriting the microbes in the world around you with your own. When you move house, your microbial aura moves too.
The team could discern the directions of these flowing microbes because they collected so many samples over such a long time. They could show that a bacterium on one person’s hand showed up later on the kitchen counter, and then on another person’s hand.
This flow is mostly one-way. Think of it like osmosis. The environment has microbes in it, but it’s a barren wasteland compared to the teeming metropolis that is your body. Unless your own partners are depleted, say by antibiotics, you rarely pick up microbes from the world around you. “Unless you have some predisposing condition that suppresses your microbiota, it’s going to wipe out anyone else’s in a very short time,” says Gilbert. “We’ve got a new study with restrooms showing that it happens in under two hours.”
That’s also because most of microbes in your home are dead or dormant. They have very little water or nutrition to work with. They’re mostly sitting there doing nothing. When you touch something, you suddenly add millions—perhaps billions—of living, thriving microbes. That new population grows like mad because you’ve also supplied it with water, oils and other nutrients from your skin. It ousts the old guard.
And these microbes that we imprint upon our homes also carry our identity. From this study and many others, it seems that everyone has a unique microbiota. We may share species and strains but the exact roll call, and the relative abundance of each member, behaves like a fingerprint. When my finger touches a surface, it leaves a unique whorl of oil and cells, but also a unique community of microbes.
There are obvious forensic applications. If you create a database of just one microbial profile per individual, you should be able to swab a doorknob and work out which of those people touched it, and (since the microbes decay over time) when they last did so. Certainly, the team could identify each of the 18 people in the study in this way. “It was never ambiguous,” says Gilbert.
That may change if you scale up to an entire city- or nation-wide population, but Gilbert thinks that the unique character of each person’s microbial community should still make them identifiable. “It’s highly likely that we’ll always be able to differentiate that,” says Gilbert. “And remember that a fingerprint tells you someone was there. A microbial fingerprint can tell you how long ago they were there, or what they were involved in—whether they’re an office worker or pig farmer.”
It’ll probably take a while for any forensic applications to come to pass. But as I talk to Gilbert over Skype, a more immediate application jumps onto his lap. “We actually got a dog on the basis of these results,” he says. He’s an adorable mix of (probably) golden retriever, collie, and other breeds. The family got him from a shelter, and the kids named him Captain Bo Diggley. Gilbert just calls him Bo.
Dogs supercharge the flow of microbes between people and their homes. If two people share a house, they also tend to share their microbes, and couples do so more than mere roommates. But if there’s a dog around, that traffic surges. Dogs also increase the microbial diversity of a home by bringing in bacteria from the outside world. In a world where the presence of bacteria is equated to filth and squalor, some people might see that as a bad thing. Gilbert saw it as a plus. We need microbes to help train our immune systems and to ensure that they develop properly. “We wanted to make sure that our kids had that capacity,” he says.
Reference: Lax, Smith, Hampton-Marcell, Owens, Handley, Scott, Gibbons, Larsen, Shogan, Weiss, Metcalf, Ursell, Vazquz-Baeza, Van Treuren, Hasan, Gibson, Colwell, Dantas, Knight & Gilbert. 2014. Longitudinal analysis of microbial interaction between humans and the indoor environment. Science http://dx.doi.org/10.1126/science.1254529