From regionalization to homogenization: Nationwide metagenomic assessment of priority pathogens and the resistome in Polish hospital wastewater.
Hospital wastewater (HWW) is a critical hotspot for the dissemination of antibiotic resistance genes (ARGs) and pathogens. This study provides the first comprehensive metagenomic characterization of HWW across Poland, analyzing 64 medical facilities across two seasons via Nanopore long-read sequencing (total of 128 HWW samples). The HWW microbiome was mostly dominated by Proteobacteria, Bacteroidota, and Firmicutes. Multivariate analysis confirmed a significant seasonal shift in the resistome. Winter samples exhibited geographic regionalization, with localized hotspots of specific ARGs, including vancomycin resistance (operon van) and carbapenemase genes (blaOXA, blaNDM). Conversely, summer samples showed a significant trend toward nationwide homogenization, characterized by a uniform distribution of ESBL genes (blaTEM, blaCTX-M) and multidrug resistance (MDR) determinants, alongside the persistence of localized clinical hotspots. Klebsiella pneumoniae emerged as a central network hub, particularly in summer, showing strong correlations with ESBLs. Quantitative genomic co-occurrence analysis revealed a functional division within dominant taxa: while environmental species like Acinetobacter johnsonii comprised the general background microbiome, clinical pathogens such as Acinetobacter baumannii served as primary vectors, showing frequent associations with high-risk ARGs. Environmental and opportunistic bacteria, such as Aeromonas spp. and Citrobacter spp., were identified as putative 'bridge hosts' associated with mobile resistance determinants and potentially contributing to HGT. The findings indicate that seasonal factors, such as increased temperature and sub-inhibitory antibiotic concentrations, may contribute to the transition from regionalized to homogenized resistance profiles, demonstrating that background resistome convergence can coexist with point-source clinical outbreaks. This seasonal "blurring" of regional boundaries positions HWW as an active vector for large-scale antimicrobial resistance (AMR) dissemination. These results underscore the urgent need for nationwide metagenomic surveillance and advanced wastewater treatment strategies within the "One Health" framework to mitigate the environmental spread of WHO priority pathogens.