The findings demonstrate how money can silently record human activities, leaving behind what are called "molecular echoes."
In April, a study identified more than 100 different strains of bacteria on dollar bills circulating in New York City. Some of the most common bugs on our bills included Propionibacterium acnes, a bacterium known to cause acne, and Streptococcus oralis, a common bacterium found in our mouths.
The research team, led by biologist Jane Carlton at New York University, also discovered traces of DNA from domestic animals and from specific bacteria that are associated only with certain foods.
A similar study recovered traces of DNA on the keypads of automated teller machines, or ATMs, reflecting the foods people ate in different neighborhoods. People in central Harlem ate more domestic chicken than those in Flushing and Chinatown, who ate more species of bony fish and mollusks. The foods people ate transferred from fingers to touch screens, where scientists could recover a bit of their most recent meals.
We don't leave only food behind. Traces of cocaine can be found on almost 80 percent of dollar bills. Other drugs, including morphine, heroin, methamphetamine and amphetamine, can also be found on bills, though less commonly than cocaine.
Identifying foods that people eat or the drugs people use based on interactions with money might not seem all that useful, but scientists are also using such data to understand patterns of disease. Most of the microbes the researchers in New York identified do not cause disease. But other studies have suggested that disease-causing strains of bacteria or virus could be passed along with our currency.
Bacteria that cause food-borne illness — including salmonella and a pathogenic strain of E. coli — have been shown to survive on pennies, nickels and dimes, and can hide out on ATMs. Other bacteria, such as methicillin-resistant Staphylococcus aureus, which causes skin infections, are found on bank notes in the United States and Canada, but the extent to which they could spread infections is unknown.
Try as we may to avoid exposure to germs, they travel with us and on us. Even if disease-causing microbes can survive in places such as ATMs, the good news is that most exposures don't make us sick.
Disease transmission linked to money is rare, and no major disease outbreaks have started from our ATMs. Although it doesn't seem common for diseases to transmit through money, there are ways we could make our money cleaner.
Researchers are working on ways to clean money between transactions. Putting older bills through a machine that exposes them to carbon dioxide at a specific temperature and pressure can strip dollar bills of oils and dirt left behind by human fingers, while the heat kills microbes that would otherwise linger.
U.S. money is still made from a blend of cotton and linen, which has been shown to have higher bacterial growth than plastic polymers. Several countries are transitioning from money made of natural fibers to plastic, which may be less friendly to bacteria. Canada has had plastic money since 2013, and Britain transitioned to a plastic-based bank note last year.
Even if our money is not directly responsible for spreading disease, we can still use the dollar's travel history to track how we spread disease in other ways. WheresGeorge.com, a website created in 1998, lets users track dollar bills by recording their serial numbers. In the almost 20 years since the site's creation, WheresGeorge has tracked the geographic locations of bills totaling more than a billion dollars.
Now, physicists at the Max Planck Institute and University of California at Santa Barbara are using data from WheresGeorge to track epidemics. Information on human movement and contact rates from WheresGeorge was even used to predict the spread of the 2009 swine flu.
Although we don't know the extent to which money allows diseases to spread, Mom's advice is probably best when handling cash: Wash your hands, and don't stick it in your mouth.
Ohm is a graduate student in biology at Pennsylvania State University. Andrew Read, Evan Pugh University Professor of Biology and Entomology at Pennsylvania State University, contributed to this article, which was originally published on theconversation.com.