A roller derby tournament seems like a brutal research environment: women crash around a rink in short skirts and skates, slamming their shoulders into members of the opposing team so that their own team’s “jammer” can lap them and score. But it’s perfect for researchers investigating how, through skin-to-skin contact, we might colonize other people with the microorganisms that colonize us.

When curious researchers armed with cotton swabs and strong stomachs sequence DNA from microorganisms gathered from our armpits, belly buttons, and various other locales, both inside and outside us, they find miniature versions of ecosystems like those in rain forests and meadows, composed of trillions of microbes. In aggregate, this invisible mass of organisms is our “microbiome,” and it makes up as much as three pounds of our body weight.

Although the bacteria that make up most of our microbiome (which also includes archaea and fungi) have likely evolved with us since the beginning of humanity, it is only since the advent of cheap, fast DNA sequencing that we’ve been able to study them in depth and begun to wonder what, exactly, they are doing here. There is growing evidence that our microbiomes can protect us from certain kinds of disease, or, if the balance between certain strains is disturbed, contribute to them.

The scientists at the roller derby tournament were investigating whether these microbiome bacteria spread from person to person when people touch—in this case, during a contact sport. To begin, they swabbed the upper arms of skaters on teams from three different cities before the competition started. The researchers found that each team had its own unique set of bacterial species thriving on players’ skin. The differences between them were so great that it was possible to tell just from a glance at a player’s skin bacteria which team she was from, recalls James Meadow, a microbial ecologist at the University of Oregon who is an author of the paper. Then the researchers took another set of samples after the teams had played each other.

They found that the complements of skin bacteria grew more similar, blurring the distinctions between the teams. The players were colonizing each other, it seems.

At first glance, it’s not such a surprising finding: after all, bacteria that make us sick are known to spread from person to person. But the study marks the first time that microbiome bacteria, not thought to be infectious, have been observed making the leap, and the implications reach beyond whether we swap benign bugs when standing close to strangers on the subway.

Even though the study focusses on the skin microbiome, it may have implications for other important microbial communities, like the ones in our guts. Gut bacteria, it turns out, are on everything. They are happiest when buried deep in our intestines, but they seem to leak out and have been found on our skin, in our chairs, in our beds—all over the place. “We think of clothes and things as being this really strong barrier, but all human microbiome studies to date have found that there’s a lot of overlap between these things, that these barriers that we think are nice and clean in our heads just don’t exist,” Meadow says. These bacteria don’t live for very long on skin and other surfaces—a matter of minutes—but they are constantly washing up there. Meadow says it would be odd if these bacteria from other people weren’t somehow making their way into our mouths. From there, they might head down to our guts.

Although it’s not clear whether other people’s gut bacteria can make it intact through the gauntlet of the alimentary tract, and whether they can establish themselves in our guts, it would help explain certain things. After a press conference two years ago, I had a long conversation with Balfour Sartor, a gastroenterologist at the University of North Carolina at Chapel Hill who specializes in inflammatory bowel diseases, like Crohn’s. These autoimmune conditions are thought to be caused in part by an imbalance of bacteria in the gut—too much of one kind, too little of another—and the genetic factors that abet this lopsided growth. Sartor told me that there was a growing stack of evidence, some published, some not, that people who had lived with inflammatory bowel disease sufferers for long periods of time had higher rates of it themselves than people in the general population. Since genetics aren’t contagious—at least not within a generation—this led him to wonder whether it was possible that the healthy roommates or spouses were being colonized, over time, by the malevolent bacteria.

While neither Sartor nor Meadow were aware of each other’s observations when I contacted them, according to Meadow, it would be only too easy for the spread of bacteria, at least, to take place. “People who live together, who are roommates, or spouses, have a more similar microbiome to their own housemates than they do to other people, say, in the same town,” he says. Sartor, for his part, notes a “concerning” 2007 study in which deliberately transmitting the gut bacteria of mice genetically predisposed to develop intestinal inflammation to healthy mice caused the healthy mice, in turn, to develop the condition.

These studies, taken together, raise the question of whether there is an infectious component in disease previously thought to be unspreadable, but it bears repeating that it is not clear whether bacteria added to our microbiomes from the environment establish themselves on or in our bodies for any length of time—and, if they do, whether they have any appreciable effect on our well-being. “When you do share microbes, we have no idea how important that is,” Meadow says. “We don’t know if they matter for your health at all.”

By Veronique Greenwood, from The New Yorker