Copper is well documented for its impressive antibacterial properties. Even Ancient Egyptians used bronze filings (an alloy of copper and tin) from their freshly sharpened swords to treat their wounds. Although we don’t carry around bronze swords anymore, we do still have copper-containing objects in our wallets: coins.
Money is probably one of the most frequently handled daily objects, and it is thought could be involved in the transmission of pathogens. To document copper’s antimicrobial ability, particularly in relation to other forms of money, Professor Johannes Knobloch and colleagues, from the University Medical Center Hamburg-Eppendorf, Germany, recently carried out tests that showed European banknotes are more easily contaminated by microbes than coins. However, the researchers were quick to point out that this did not completely stop coins from harboring some bacteria, and remained a potential source of contamination after 24 hours.
For the study, the researchers contaminated three denominations of euro coins (5 cent, 50 cent, €1) containing at least 75 percent of copper and one denomination of banknote (€5) made of cotton fiber, with one of two well known infection-causing bacteria (Staphylococcus aureus and Enterococcus faecium). Ceramic tiles were also contaminated, in order to be used as a control surface. After contamination, the surfaces were stored for 24 hours before the number of bacterial cells were counted again.
From nine independent experiments (four using E. faecium and five using S. aureus), the researchers noted that after 24 hours the banknotes and control surface displayed similar bacterial cell counts. On the coins, however, after the 24-hour period, S. aureus was reduced by between 98.7 and 99.5 percent (dependent on coin type) and E. faecium by 96.8 to 99.0 percent.
“In contrast to a 5 euro banknote, copper-containing coins display a detectable antimicrobial activity,” the authors conclude in their study. “However, in most experiments, bacteria were not completely eliminated from the coins. Therefore, even coins might act as vectors for the transmission of microbes.”
As the research was due to be presented at the European Congress on Clinical Microbiology and Infectious Diseases (ECCMID), which has been canceled due to the Covid-19 pandemic, the study has yet to be peer-reviewed. Therefore, as some details of the study are not fully addressed, such as how the number of bacteria found on coins compares to the control surface, other scientists recommend that the findings should be interpreted with caution.
One point that should be stressed about this study, is that the microorganisms used for contamination were bacteria, not viruses. Thus, with regards to the SARS-CoV-2 virus, which causes COVID-19, the findings cannot be directly extended.
“A very important (non-statistical) point at the present time is that the micro-organisms involved were two different bacteria, and not a virus, so the findings tell us nothing practical about how a virus such as SARS-CoV-2 might survive on coins or banknotes,” Kevin McConway, an Emeritus Professor of Applied Statistics, at The Open University, who was not involved in the research, said.
"If you're touching items that someone else has recently handled, be aware they could be contaminated and wash your hands,” James Lloyd-Smith, co-author of the NEJM study and UCLA professor of ecology and evolutionary biology, said.