The germ is MRSA, or methicillin-resistant Staphylococcus aureus - a hard-to-treat bug once rarely seen outside of hospitals but that increasingly is spreading in ordinary community settings such as schools, locker rooms and gyms.
The germ causes nasty skin infections as well as pneumonia and other life-threatening problems. It spreads mostly through human contact. Little is known about environmental sources that also may harbour the germ.
Finding it at the beach suggests one place that people may be picking it up, said Marilyn Roberts, a microbiologist at the University of Washington in Seattle.
"We don't know the risk" for any individual going to a beach, she said. "But the fact that we found these organisms suggests that the level is much higher than we had thought."
She presented results Saturday at an American Society for Microbiology conference in California. Last year, her team reported finding a different type of bacteria, enterococci, at five West Coast beaches. And earlier this year, University of Miami researchers reported finding staph bacteria in four out of 10 ocean water samples collected by hundreds of bathers at a South Florida beach.
Many communities also commonly restrict bathing at beaches because of contamination with fecal bacteria.
In the new study, researchers tested 10 beaches in Washington along the West Coast and in Puget Sound from February to September 2008. Staph bacteria were found at nine of them, including five with MRSA. The strains resembled the highly resistant ones usually seen in hospitals, rather than the milder strains acquired in community settings, Roberts said.
No staph was found in samples from two beaches in southern California.
People should not avoid beaches or be afraid to enjoy them, scientists say.
"It's probably prudent to shower when you come out" to lower the risk of bacteria staying on the skin, said Dr. Lance Peterson, a microbiologist at NorthShore University Health System in Illinois.
"Make sure you get all the sand off," and cover any open cuts or scrapes before playing in the sand, Roberts added. Digging in the sand or being buried in it seems to raise the risk of infection, she said.