The phrase "dirty money" is more literal than you might have thought, a study of how much bacteria lurks on the average dollar bill suggests.

Analyzing dollar bills supplied by a New York bank, researchers found they harbored almost 3,000 varieties of bacteria, mostly types found on human skin.

While most were harmless varieties causing minor conditions such as acne, researchers at New York University say they found antibiotic-resistant kinds such as methicillin-resistant Staphylococcus aureus, known as MRSA.

That suggests bills could be serving as a vehicle by which such antibiotic-resistant varieties to move around the country, researcher Jane Carlton says.

 "We are finding viable bacteria that can be taken from paper currency," she says. "That means that money could function as a form of transmission."

Since cash is carried in wallets or purses held close to a person's body, the resulting warmth provides an ideal environment for microbes to thrive, the researchers noted.

There was also a seasonal factor in the kinds of bacteria found on bills, they said, with an increase of pneumonia microbes found on paper currency in winter time.

Despite the dramatic nature of the findings, the researchers said, people should avoid overreacting about their money.

"Microbes are so important, are very ubiquitous and they surround us all the time," Carlton said. "We did find certain microbes that we might be a little concerned about, but that doesn't mean that people should be unduly concerned."

"We're not trying to be fear mongers, or suggest that everyone goes out and microwave their money," said Carlton, who is head of genome sequencing work at the university. "But I must admit that some of the $1 bills in New York City are really nasty."

Some experts have suggested printing money on something other than paper could alleviate the situation, and some countries including Canada have begun to print their currently on plastic polymer film.

Whether that can help remains up in the air; while one study found less bacteria growing on the plastic bills than on paper ones, another study found bacteria, once on plastic bills, survived longer than they did on paper currency.