We wanted to document and quantify the extent of such contamination during heavy rainfall. This is challenging, because such events are relatively rare, and you have to be on site when they happen. For instance, even though these events occurred in Münchwilen on 118 calendar days in 2019, most did not last long. During heavy rains, many other potential sources of antibiotic resistance contamination enter the river, complicating our research further. This includes surface runoff that transports soil into the river, such as from fields and meadows that may have been manured. Sediment within the river is also resuspended by the faster flow.
Using modern sequencing techniques to distinguish microbes from different sources, we disentangled these inputs. Our results show that the sewage overflow, so-called ‘wastewater bypass’, was clearly the major source of antibiotic resistance in the river. In many wastewater plants, including the one in Münchwilen, the bypass at least undergoes a sedimentation process before it is discharged to the river. However, post-sedimentation levels of antibiotic resistance were still very high.