Water as a reservoir of last-resort drug resistance: The presence of mcr genes in wastewater and rivers demonstrated by the entire drainage basin

The rapid emergence and dissemination of plasmid-mediated mcr genes have raised significant public health concerns. Given the spread of colistin resistance across the natural environment, humans, and animals, the water environment serves as a crucial connecting medium. To address critical gaps in the literature on the dissemination of mcr variants in environmental samples, we prepared the first report on the prevalence of mcr genes in wastewater and river water samples in Poland. We designed the study at the river-basin scale to provide comprehensive insight into colistin resistance mediated by mobile mcr genes in the aquatic environment. Moreover, this is the first study to examine seasonal variation in mcr genes, conducted simultaneously on wastewater and river water samples. The research involved collecting wastewater from 17 wastewater treatment plants (including facilities that receive hospital wastewater) and river water at 8 points along the Pilica River across four seasons. We demonstrated that mcr -1, mcr -4, and mcr -5 were detected in all samples, both wastewater and river water, with mcr -5 the most abundant. The wastewater treatment process was completely ineffective at eliminating mcr -5 genes. In addition, significant differences in the prevalence of mcr -1, mcr -4, and mcr -5 were observed between warmer and colder seasons, indicating a seasonal effect on the dissemination of colistin resistance genes in environmental samples. The present findings help clarify the role of wastewater and rivers in the spread of last-resort drug resistance genes and their significance for public health.