Cefiderocol-resistant Aeromonas with expanded Resistomes in German hospital wastewater: Phenotypic and genomic evidence from the environment-clinical Interface.

Hospital wastewater is a key interface between clinical and environmental reservoirs of antimicrobial resistance, fostering selection and horizontal gene transfer. Aeromonas spp. are aquatic opportunistic pathogens with highly plastic genomes and are increasingly recognized as potential intermediaries in resistance dissemination. We compared 72 cefiderocol-selected Aeromonas isolates recovered from untreated hospital wastewater collected at six tertiary care hospitals across Germany with 62 clinical isolates from patients with intestinal and extraintestinal infections, to characterize cefiderocol susceptibility, resistome composition, and genomic mobility features. Pangenome analysis revealed an open genome structure comprising 21,364 gene clusters, with a core genome of 2486 genes and a large cloud gene pool (15,612 clusters present in <15% of isolates), highlighting extensive genomic plasticity. Resistance phenotypes diverged markedly: cefiderocol-selected wastewater isolates exhibited high resistance rates to multiple clinically relevant agents - ciprofloxacin (93.1%), aztreonam (81.2%), and trimethoprim-sulfamethoxazole (38.9%), whereas clinical isolates remained largely susceptible overall (<10%). Under iron limitation, siderophore production increased in both cohorts; however, in the presence of cefiderocol it remained robust in wastewater isolates while being suppressed in clinical isolates. Comparative genomics showed that wastewater isolates carried substantially expanded resistomes (mean 13.8 ARGs; range 2-27) relative to clinical isolates (mean 2.6; range 1-11), including enrichment of clinically relevant β-lactamases and carbapenemases. This resistance burden coincided with a larger and more transmissible plasmidome and a high insertion sequence load. Notably, extensive plasmid-backbone homology was detected between Aeromonas and co-occurring cefiderocol-resistant Enterobacterales isolated from the same wastewater samples, highlighting interspecies gene flow at the hospital-environment interface. Together, these findings identify hospital wastewater as a reservoir and convergence point for highly resistant, mobilome-enriched Aeromonas subpopulations captured under cefiderocol selection, supporting Aeromonas as a One Health sentinel and emphasizing the value of wastewater-based surveillance for tracking mobile resistance determinants bridging environmental and clinical compartments.