Whether they're skating shoulder-to-shoulder to block the other team or laying each other out with body checks, roller derby players have a lot of skin-to-skin contact. That contact spreads more than sweat, according to a new study. Researchers have found that players come into a tournament bearing a team signature of bacteria on their shoulders-but leave sharing microbes with their opponents.

The study adds to knowledge of how microbes colonize our skin and how much our microbial communities-or microbiomes-change when we contact other people or surfaces, whether it's a doorknob at home or medical equipment in a hospital. "This is an important step forward in our understanding of how we share our microbiomes when we interact with other people," says Jack Gilbert, an environmental microbiologist at Argonne National Laboratory in Chicago, Illinois, who was not involved in the work.

The skin microbiome plays key roles in health and disease: It can carry pathogenic bacteria, but most species are harmless and may even contribute to our health. A study last year, for instance, found that good-guy skin bacteria help train the immune system to fight pathogens. And just last month, researchers showed that microbes on the face can protect against acne.

Yet, scientists know very little about how our resident bacteria come to live with us, or how these populations change over time. "There are certain [microbes] we all have, and certain things that are unique to individuals, but we really have no idea where we acquire these in our lifetime," says James Meadow, the study's lead author and a microbial ecologist at the University of Oregon in Eugene.

Roller derby provided an ideal setting to study the microbial effects of skin-to-skin contact. It is, after all, a full-contact sport, and senior author Jessica Green, who is also a microbiologist at the University of Oregon, was once a roller derby player herself.

In flat track roller derby, a pack of eight blockers (four from each team) plays offense and defense at the same time. They aim to help their point scorer, known as the jammer, through the pack, while stopping the other team's jammer from getting through. The more times a jammer laps the pack, the more points she gets. The players interfere with each other by means of legal hits, coming mainly from the hips and shoulders. (Punching and elbowing are outlawed.) This means that a player's shoulders often contact those of her opponents. Teammates also contact each other as they skate in formation, clogging up the opposing jammer's path through the pack.

During a full 60-minute bout, the researchers hypothesized, the players' shoulders would have plenty of opportunities to swap skin bacteria. So they sampled skaters' shoulders before and after two bouts in a tournament hosted by local league Emerald City of Eugene.

"These teams came to the tournament from different places, and we were kind of shocked to find out that they had a unique team microbiome," Meadow says. "In other words, we could have picked a player out at random, before she played, and I could tell you which team she played for just by sampling the bacteria on her upper arm."

 After each bout, though, the samples told a different story: the teams' microbiomes converged, having more species in common. For example, before Emerald City played Silicon Valley, members of the two teams shared 28.2% of their bacterial communities. After the bout, the overlap was up to 32.7%, the team reported Tuesday in PeerJ.

 It's possible that the microbiomes became more similar because heat and perspiration made the players' skin more hospitable to certain bacteria. But the researchers were able to "kind of rule out exercise and sweating," Meadow says, by considering factors such as the time it takes bacteria to reproduce. At 20 minutes per generation, a mere three generations in an hour of playing time is unlikely to have caused the dramatic shift they saw. Gilbert agrees with the researchers' interpretation that physical contact, not sweating, probably caused the shift.

This work didn't study what happened to the players' microbiomes in the days after the tournament, and more work is needed to understand those longer-term changes, Meadow says. "What we're interested in is how our choices affect the bacteria that associate with us," says Meadow, whose research focuses on how microbes live in the "built environment," which includes everything in buildings, such as the ventilation systems and the chairs we sit in. "This study highlights that our interactions with people around us do appear to change our microbiome," he says. "When you ride to work on the subway and bump arms with someone, is that small contact enough to share something?"

Learning how microbes travel is also important for understanding the spread of disease. In the past, Meadow says, our knowledge about the skin microbiome came from medical studies attempting to retrace the steps of a single pathogen, for example through a hospital during an outbreak. But research like this, he says, "gives us an opportunity to look at how healthy people share their whole communities of microbes, because that's really what's going on. Pathogens are a very small part of what we're carrying around on us."

Microbiome research, Gilbert says, needs to map the "highways and byways" of how microbes travel, because pathogens could travel the same routes as healthy bacteria. "It's very important that we understand what those routes are," he says. "What we really need is a predictive road map, a TomTom for pathogens."