Dirty money harbors bacterial dangers.

There's a new reason for laundering money. Dollar bills support a rich flora of bacteria, some of them infectious, say research physicians Theodore W. Pope and Peter T. Ender of the Medical Center of Wright-Patterson Air Force Base in Ohio.

They worked with Michael A. Koroscil at Beavercreek (Ohio) High School, who originated the idea for a science-fair project. The group counted the bacteria living on 68 dollar bills collected from people in line at a high school sporting event and a grocery store.

The researchers incubated each bill in nutrient broth for up to a day and then grew any bacteria in culture dishes to identify them. More than half the bills hosted bacteria that commonly infect people in hospitals or those who have depressed immune systems, the team reported. Five of the bills contained a bacterium that can sicken healthy people, either Klebsiella pneumonia or Staphylococcus aureus. The researchers identified a total of 93 different types of bacteria living on the bills, and two-thirds of the bills had at least one type.

That paper money harbors bacteria wasn't surprising, says Ender. The researchers didn't expect, however, to find so many dollar bills contaminated with such a diversity of organisms.

Pope notes that the findings are important because "unlike other frequently touched surfaces that harbor bacteria, such as door knobs and computer keyboards, dollar bills move throughout the community--in fact, throughout the country and the world." Generally ignored as conveyors of microbes, bills could be a much more mobile source of infections than other contaminated objects.

"It doesn't mean we need to go out and wash the money or change the way we do business," says Pope. The findings just reinforce the need for proper hygiene, such as washing hands frequently, especially before eating, he says.