Surfers are uniquely exposed to the ocean, ingesting more water than just about anyone (ten times more than sea swimmers even, according to estimates). This apparently makes them good bell-weathers of marine health: Whatever is in their guts is a reliable indication of what’s in the water.

That’s why researchers in Britain are taking swabs of surfers’ rectums: They’re a (dirty) window on the ocean, so to speak. “We’re looking for bacteria that live inside people’s guts that are resistant to antibiotics that are really important in clinical medicine to treat bacterial infections,” says Anne Leonard, a researcher with the University of Exeter.

Leonard launched a campaign recently with the environmental action group Surfers Against Sewage. The goal is to find beach bums to come forward to offer samples. Happily, plenty seem willing so far. “We have received an overwhelming number of expressions of interest from surfers and non-surfers alike,” Leonard says.

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The serious point of the study is to look at one of the ways humans come into contact with drug-resistant bacteria. At the moment, Leonard says, researchers know more about settings like hospitals, food facilities, and airplanes, and less about the natural environment, including the ocean. In previous research, Leonard estimated that 6.3 million English and Welsh bathers were exposed to at least one drug-resistant bacteria in 2012, and that 0.12% of the E. coli was resistant to third-generation cephalosporins, an important class of antibiotics.

The bigger question is how bacteria might move from swimmers and surfers into the wider population. “We suspect that if swallowed bacteria survive in the human body, they can live in the gut,” Leonard says. “There is the possibility that these bacteria can then be spread to other members of the community. This could be a problem if people with suppressed or compromised immune systems acquire resistant bacteria because they are more susceptible to developing infections.”