Holidaymakers should shower after swimming in the sea to reduce their chances of picking up the superbug, MRSA, according to scientists.
The warning follows one of the first major studies into dangerous microbes that bathers might encounter during a trip to the seaside.
Researchers found that people who went swimming at a popular beach in Florida had more than a one-in-three chance of coming into contact with the Staphylococcus aureus microbe. A small proportion of these were the potentially life-threatening drug-resistant strain, MRSA.
The bugs get into seawater when they are washed off the skin of people who may be unaware they are carrying the infection.
"Staphylococcus are shed by individuals into the waters and if you do go into these waters you are likely to be exposed," said Lisa Plano who led the research at the University of Miami.
In Britain, drug-resistant MRSA has largely been a problem confined to hospitals, but in recent years, health officials have noticed a rise in what are called community-acquired MRSA infections. These often affect young, active people who have never spent time in hospital.
Estimates suggest that between 20% and 40% of people in Britain carry MRSA, mostly on the skin or in the nose. The disease tends to cause serious infections only when it gets into deep wounds or contaminates medical implants, such as artificial hip joints and heart pacemakers.
"As infections with these organisms are on the rise, it is becoming increasingly more important to determine all the sites or sources where these germs might survive and be shared among people," Plano said.
For the study, the scientists chose a beach that was far away from any sewage outlets. They recruited 1,303 beachgoers and divided them into two groups. The first sat on the beach for 15 minutes, while the second ventured into the water and took sterilised jugs with them to collect water samples.
Analysis of the water found 37% of swimmers were in contact with Staphylococcus aureus, and 3% of these were the drug-resistant strain. Plano, who presented the results at the American Association for the Advancement of Science meeting in Chicago yesterday, said they pointed to bathing as a possible risk factor for picking up MRSA.
"The majority of the isolated MRSA were those likely to be of the more aggressive variety," she said. "This exposure might lead to colonisation or infection by water-borne bacteria, which are shed from every person who enters the water."
She said there was no reason to avoid swimming in the sea, but recommended people take precautions to reduce the risk of spreading, or picking up the bugs, by showering before and after going for a swim.
