SBP 32 - The (Microbial) Value of Your Money

In a post-COVID world, people have become more vigilant about things they are exposed to. Frequent disinfecting of surfaces and hands have become habits that many have adopted. Despite the extra precautions taken, many still overlook frequent contaminants that are utilized every day. Studies have found that currency, whether it be bills or coins, harbors several microorganisms. However, many do not recognize the need to clean their money or are hesitant to do so due to fear of damaging them. To bring awareness to the pathogenic potential of money, this research was done to identify the endemic microorganisms found in Canadian polymer notes. 

In this investigation, five-dollar polymer notes were used to represent Canadian currency, as previous studies have shown that lower denominations possessed the highest microbial load. Eight polymer notes were taken from circulation and swabbed. The cotton swabs were then diluted and plated. After incubation, the plates revealed the presence of a variety of microorganisms. A total of thirteen colonies of different appearances were isolated. These colonies were stained with gram stain or lactophenol blue stain in order to differentiate them into gram-positive, gram-negative, or fungi. 

Most of the isolated microorganisms were gram-positive, with five gram-positive cocci and four gram-positive bacilli. Two gram-negative bacilli were isolated, as well as one fungi and one yeast. Through the use of biochemical tests and selective media, the microorganisms found were identified to be Bacillus spp., coagulase-negative Staphylococcus spp., Micrococcus spp., Penicillium spp., and Candida spp, with specific identification of Yersinia pseudotuberculosis, Proteus penneri, Bacillus cereus, and coagulase positive Staphylococcus aureus. Depending on microbial load, these isolated bacteria and fungi may cause illness; thus, investigation on factors that contribute to their presence in currency may be medically relevant.

An important factor in the microbial potential of currency notes is their composition. Many countries, like the United States, use a linen-cotton mixture for their currency notes, while other countries use unique blends, such as Japan’s Mitsumata paper. Since 2011, Canada has adapted to use polymer material for their currency notes. Though these notes were designed with security and durability in mind, a comparison was done in this research to determine whether this material is the safest choice. This was accomplished by using Staphylococcus aureus found by the researchers through the microbial assay of Canadian polymer notes. 

           In this research, polymer, cotton-linen, and Mitsumata currency notes were used. These notes were first sterilized with 265nm UV light to decrease microbial load prior to inoculation with the colony of interest. Post-sterilization, one cotton-linen note served as the representative of the baseline count as well as to prove absence of competing organisms. Two sets of currency notes were inoculated with approximately 1.8x10⁸ colony-forming units by instilling 0.150ml of 6.0x10⁸ cfu/ml mixture on each side. After drying for 30 minutes, one piece of each currency was then placed into a WhirlPak bag with 20ml PO₄, which was plated and incubated for enumeration. To replicate the typical storage of currency, the inoculated notes were put into a single wallet and stored at room temperature. After 72 hours of storage, the enumeration method was repeated for this set of currency notes.

Enumeration revealed that there was a 99.9% decrease from the initial estimated concentration of bacteria in all currencies. However, upon enumeration of currency after storage for 72 hours, it was noted that bacterial numbers increased in polymer notes by 12%, and in Mitsumata notes by 1100%. Interestingly, the linen-cotton blend had a 49.23% decrease. 

Through this investigation, it was determined that many concerning microorganisms are present in the money we use every day. Despite their presence, this study showed that only 0.01% of the initial inoculated bacteria persisted in the currency notes. However, it is of concern that there was an increase in bacterial load after storage. Further investigation must be carried out to determine how the population growth would trend with increased storage time. Future investigations may also include the possibility of factors such as note to note bacterial transference when currency notes are stored together.