It seems like common sense, and now science backs it up: If you swim in water with a lot of people, it appears you're more likely to run into their disease-causing bacteria than at a beach with fewer people.

Microbiologist Roger S. Fujioka, of the University of Hawai'i's Water Resources Research Center, conducted studies on the water at beaches after doctors expressed concern that people were coming down with staph infections after swimming at popular beaches.

"There's lots of people that go swimming in coastal water and come out with staph infections," he said.

Fujioka tested the water for the bacterium that causes staph infections, Staphylococcus aureus, and found that the numbers compare nicely with the number of people at the beaches.

Busy Waikiki Beach, for example, showed about 20 staph bacteria per 100 milliliters of water. By contrast, the less congested beaches of the Wai'anae Coast showed between 0 and 5.

"It's correlated with the density of swimmers," he said.

Fujioka said it is clear that the staph counts are associated with swimmers, not with things like sewage spills.

"Thirty percent of people have staph bacteria on the skin or in their nose" and it seems from the results that those bacteria wash off and remain in the water for some time.

"We tracked it at Kuhio Beach, when people left the beach it (bacteria levels) dropped off, but it didn't go away," he said.

It seems clear that the amount of flushing action in the nearshore water would reduce the bacteria count. The Kuhio Beach area he tested does not get great flushing action. Areas along the Wai'anae Coast can have more water movement, and that could help keep bacteria counts lower there, he said.

People generally get staph infections when bacteria infect an open wound. That can be an existing wound, or a scrape or cut acquired while someone is in the water. Wounds can be more readily acquired after the skin is softened by the water — from scrapes on the sand, kicking a hard surface or even getting an abrasion from a surfboard.

Fujioka said his research doesn't prove that staph infections from nearshore water are caused by bacteria from other swimmers, but the evidence points that way. Another theory is that individuals have the bacteria on the skin, and that they infect themselves when they get a wound.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766 or jant@honoluluadvertiser.com. Or call him at (808) 245-3074.