Banknote paper is manufactured from cotton fibre, which gives the paper its strength, durability and distinctive feel [10]. The cotton is sometimes mixed with linen, abaca, or other textile fibres. Unlike most printing and writing paper, banknote paper is infused with polyvinyl alcohol or gelatin to give it extra strength. Polymer (or plastic) banknotes were developed to improve durability and prevent counterfeiting through incorporated security features, such as optically variable devices that are extremely difficult to reproduce [11]. Romania, Israel, Malaysia and some other countries have adopted these polymer-containing banknotes [12]. Probably, the content of banknotes is also a factor that affects the survival of bacteria on the banknotes.

Our experiments showed that the polymer structure of the Romanian Leu banknote allows growth and transmission of multi-drug resistant pathogens. This, in theory, could contribute to the transmission of microorganisms within the Romanian community. Countries using polymer-based banknotes should take this into consideration, especially, if a currency is not exclusively used within one country, such as the Euro and US Dollar. Despite prolonged survival of E. coli on the Euro (up to 6 hours), transmission to the volunteers did not succeed. In contrast, US and Canadian Dollar showed significant and prolonged carriage of MRSA. S. aureus was transmissible from the US Dollar and thus indicates, that banknotes may play a role in the transmission of MRSA within the community. The Croatian Kuna was found to, unexpectedly, not allow growth of any of the multi-drug resistant microorganisms tested. While we could not find further information on the content and make-up of this currency, it could be interesting for other countries who want to eliminate banknotes as a source of bacterial transmission. Gram-positive and –negative microorganisms, including staphylococci, bacilli, and various enterobacteriaceae, have been found on banknotes in multiple countries, such as in Iran [8] and Turkey [12].

Money is frequently touched during daily life. The observed differences between the various currencies were unexpected and were not based on different climate conditions or hygiene levels, since all experiments were performed at the same conditions, using previously sterilized banknotes. While further studies would be needed to establish transmission of multi-drug resistant microorganisms through contact with money, or experiments show that this may be a potential pathway, especially in countries that use polymer-based banknotes.