By analyzing wastewater from five European cities, including Bologna and Rome, an international research team identified over 2,300 bacterial species, more than 1,300 of which were previously unknown. The results, which were published in Nature Communications, contribute to the understanding of the "good" and "bad" microbial ecosystems with which we interact on a daily basis without our awareness. These ecosystems have the potential to facilitate the dissemination of antibiotic-resistant microorganisms. The background is that of the One Health paradigm, which acknowledges the inextricable link between human, animal, and ecosystem health. Wastewater is a critical instrument for monitoring the condition in major urban areas. In fact, the European Union intends to establish monitoring systems in the treatment facilities of all cities with a population exceeding one hundred thousand beginning in 2025. With a two-year longitudinal study, the researchers examined and studied the microbiological content of wastewater from seven treatment plants in five European cities: Copenhagen, Rotterdam, Bologna, Rome, and Budapest. Overall, metagenomic data from 2,332 bacterial species were found, 1,334 of which had never been described previously. Seasonality emerged as a key difference among cities: Rotterdam and Copenhagen exhibited significant annual variations in bacterial communities, while cities like Bologna showed much greater stability. This highlights diverse dynamics in bacterial development and evolution influenced by a range of factors.