When competitors in the contact sport of roller derby bump, slam and hip-check each other in order to score points, they exchange bacteria from the skin-to-skin contact and little by little lose the unique bacterial signature that identified them before the bout.

That's according to a new U.S. study that tracked three roller derby teams during a tournament -- testing their skin bacterial count before and after matches.

The research provides new insights into how the germs or microbes on our skin change when we interact with other people – information that could be vital as populations increase and humans come into closer and closer contact in coming decades.

  • Scroll to the bottom to watch a video about the project

Researchers led by James Meadow, a post-doctoral research associate at the University of Oregon in Eugene, took swabs from the exposed upper arms of players on three teams: the Emerald City Roller Girls from Eugene, Ore., the DC Roller Girls from Washington, D.C., and the Silicon Valley Roller Girls from San Jose, Calif.

The upper arm is typically kept bare by roller derby players before, during and after a bout, and is the area most likely to come into contact with other players – making it a perfect Petri dish for researchers.

Swabs taken before each bout showed that athletes shared a "microbial fingerprint” unique to their team. That common set of bacteria could be because the players live in the same geographic area, train together, spend time together socially and even live together in some cases.

But after a bout, researchers found the bacterial boundaries had blurred significantly between the two competing teams as players traded germs from the near-constant skin-to-skin contact. It became much more difficult to tell the teams apart based on their bacterial swab.

"Although each team retained their microbial fingerprint, we found that team microbial communities became more similar to one another after players competed in a bout," said the study, published recently in the journal PeerJ.

Researchers acknowledged there could be three possible explanations:

  • The athletes were all exercising for the same amount of time in the same environment, which can result in a specific set of microbes;
  • Players were all picking up similar microbes already present in the track environment;
  • Or, players were exchanging germs due to the close physical nature of the sport.

The first hypothesis was eliminated because the 60-minute matches simply weren't long enough to result in such significant changes to skin bacteria (also known as “microbiome”).

The second option was ruled out because swabs taken from the floor of the track showed a very different germ network than that found on the players' skin, either before or after a bout.

That left just one possible explanation:

"Given that the proportion of (bacteria) shared between competing teams increased after both bouts, but not between non-competing teams, human-to-human contact is the most parsimonious interpretation for the significant changes in skin microbiome we observed," the study said.

The researchers acknowledged that humans trade germs all the time -- whether by shaking hands, kissing, hugging or any other form of skin-to-skin contact. And for the most part it’s a harmless exchange. In fact, the study said, there is evidence that we also exchange immune cells with those we come into physical contact with, which may help protect us from virus or infection.

But it's not always harmless. Superbugs such as methicillin-resistant Staphylococcus aureus (MRSA) can be transmitted through physical contact between athletes, or even through shared gym towels -- and can be fatal.

As the world's population increases and people begin living in closer and closer quarters, it will become increasingly important to understand the risks of person-to-person contact, the study said.

"A thorough comprehension of the drivers of the skin microbiome is still emerging; novel approaches to studying our skin ecosystems will likely have lasting implications for health care, disease transmission, and our understanding of urban environment microbiology," the study concluded.

Below is the video "Talk Derby to Me" about the project:

Talk Derby To Me from Tristan Wheelock on Vimeo.