Scientists in the United Kingdom analyzed about 300 people and concluded that surfers harbor a lot more bacteria in their guts that are resistant to antibiotic medication, compared to people who swim in the sea but don’t surf.

Led by Dr. Anne Leonard from the University of Exeter Medical School, the researchers recruited 150 surfers and 150 non-surfers, all of whom frequent the coastline of the U.K. Since the intention was to test for the presence of bacteria in their guts, the study’s participants were all given rectal swabs to collect fecal samples.

After eliminating some samples for contamination and other causes, Leonard and her team were left with swabs from 143 surfers and 130 non-surfers. Of the first group, a total of 13 — about 9 percent — had their guts colonized by strains of E. coli that are resistant to antibiotics. In contrast, only four people from the latter group — roughly 3 percent — had the same bacteria living in their stomachs. So for every non-surfer with untreatable E. coli bacteria, there were three surfers.

Regular surfers were also found four times as likely to be carriers of bacteria with mobile genes that make them resistant to antibiotics — genes that can be passed between bacteria.

“Antimicrobial resistance has been globally recognised as one of the greatest health challenges of our time, and there is now an increasing focus on how resistance can be spread through our natural environments. We urgently need to know more about how humans are exposed to these bacteria and how they colonise our guts. This research is the first of its kind to identify an association between surfing and gut colonisation by antibiotic resistant bacteria,” Leonard said in a statement Sunday.

Surfers swallow about 10 times as much seawater as those who swim in the sea. The sea is the recipient of much of humanity’s waste, including domestic sewage, industrial pollutants and agricultural runoff, all of which are potential sources of harmful bacteria.

According to the World Health Organization (WHO), the increase in antibiotic resistance among microbes is “one of the biggest threats to global health, food security, and development today.” It is making treatment of many diseases more difficult, even when it was a relatively simple process in the recent past. Some of these diseases include gonorrhea, pneumonia, salmonellosis and tuberculosis.

“Antibiotic resistance is rising to dangerously high levels in all parts of the world. New resistance mechanisms are emerging and spreading globally, threatening our ability to treat common infectious diseases. A growing list of infections … are becoming harder, and sometimes impossible, to treat as antibiotics become less effective,” WHO says on its website.

The study, which used the antibiotic cefotaxime for testing E. coli resistance, appeared Sunday in the journal Environment International.

Dr. Will Gaze of the University of Exeter Medical School, who supervised the research, said in the statement: “We are not seeking to discourage people from spending time in the sea, an activity which has a lot of benefits in terms of exercise, wellbeing and connecting with nature. It is important that people understand the risks involved so that they can make informed decisions about their bathing and sporting habits. We now hope that our results will help policy-makers, beach managers, and water companies to make evidence-based decisions to improve water quality even further for the benefit of public health.”