Scientists at the University of Exeter warned of the danger after analysing data from beaches across England and Wales in 2012.
They focused on a strain of the Escherichia coli (E.coli) bug resistant to an important class of antibiotics called third-generation cephalosporins, or 3GCs.
Even though only 0.12% of the E.coli found in coastal waters and rivers running onto beaches belonged to the strain, this presented a “potential risk”, said researchers.
Surfers and swimmers were likely to be most exposed due to swallowing water.
The study showed people’s risk of exposure to resistant bacteria was linked to water quality. This was said to demonstrate the importance of the EU bathing water directive to ensure quality standards.
The findings are published in Environment International.