Humans interact constantly with surfaces and associated microbial communities in the environment. The factors shaping the composition of these communities are poorly understood: some proposed explanations emphasize the influence of local habitat conditions (niche-based explanations), while others point to geographic structure and the distance among sampled locations (dispersal-based explanations). However, the relative roles of these different drivers for microbial community assembly on human-associated surfaces are not clear. Here, we used a combination of sampling, sequencing (16S rRNA) and culturing to show that the composition of banknote-associated bacterial communities varies depending on the local collection environment. Using banknotes collected from various locations and types of shops across Switzerland, we found taxonomic diversity dominated by families such as Pseudomonadaceae and Staphylococcaceae, but with banknote samples from particular types of shops (especially butcher shops) having distinct community structure. By contrast, we found no evidence of geographic structure: similarity of community composition did not decrease with increasing distance among sampled locations. These results show that microbial communities associated with banknotes, one of the most commonly encountered and exchanged human-associated surfaces, can reflect the local environmental conditions (in this case, the type of shop), and the signal for this type of variation was stronger than that for geographic structure among the locations sampled here.

In daily life, currency is one of the most frequently and actively exchanged commodities among humans. We collected banknotes from 50 locations in Switzerland (10 towns, five different shops in each town), with the aim to investigate the drivers behind variations in the bacterial communities on Swiss banknotes, specifically to assess if the microbiome associated with banknotes differs with geographic distance and/or the type of shop the banknote originated from (collection environment).

Each of the 50 banknotes was processed as follows: (1) we swabbed front and back of the each banknote with one sampling swab; (2) we plated a dilution of the sample on chromatic agar; (3) we extracted DNA from each swab tip; (4) we performed 16S rRNA amplicon sequencing on extracted DNA samples.