Researchers at the University of Exeter in the UK asked 300 people, half of whom regularly surf the UK's coastline, to take rectal swabs.
Surfers swallow ten times more sea water than sea swimmers, and scientists wanted to find out if that made them more vulnerable to bacteria that pollute seawater, and whether those bacteria are resistant to an antibiotic.
Scientists compared faecal samples from surfers and non- surfers to assess whether the surfers' guts contained E coli bacteria that were able to grow in the presence of cefotaxime, a commonly used and clinically important antibiotic.
The study, published in the journal Environment International, found that 13 of 143 (9 per cent) of surfers were colonised by these resistant bacteria, compared to just four of 130 (3 per cent) of non-surfers swabbed.
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That meant that the bacteria would continue to grow even if treated with cefotaxime.
Researchers also found that regular surfers were four times as likely to harbour bacteria that contain mobile genes that make bacteria resistant to the antibiotic.
This is significant because the genes can be passed between bacteria - potentially spreading the ability to resist antibiotic treatment between bacteria.
"We urgently need to know more about how humans are exposed to these bacteria and how they colonise our guts," said Leonard, who led the study.
"This research is the first of its kind to identify an association between surfing and gut colonisation by antibiotic resistant bacteria," he added.
Despite extensive operations to clean up coastal waters and beaches, bacteria which are potentially harmful to humans still enters the coastal environment through sewage and waste pollution from sources including water run-off from farm crops treated with manure.
This would mean that infections such as pneumonia, tuberculosis, blood poisoning, gonorrhoea, and food and waterborne diseases could be fatal.
It would also mean that it would no longer be possible to use antibiotics to prevent infections in routine medical procedures, such as joint replacements and chemotherapy.
"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," said Will Gaze, of the University of Exeter.
"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," he said.
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