Paired <i>in situ</i> and molecular analyses identify mechanisms of pathogen persistence within environmental communities

Abstract Natural environments are a key reservoir for many opportunistic pathogens, yet mechanisms enabling persistence within complex microbial communities remain poorly defined. Current methods do not adequately link molecular and functional understanding of bacterial persistence to the activities of bacteria within their natural microbial community. Here we combine in situ culture-independent techniques, including metagenomics and metatranscriptomics, with isolate-level functional genomics and mutant studies to understand how opportunistic pathogens from the genera Escherichia , Klebsiella , and Enterococcus survive in urban-adjacent freshwater ecosystems. . Our results show that potential pathogens from each of these genera are both present and active within freshwater microbial communities. We further investigate a mouse-virulent isolate of the E. coli UPEC lineage ST73 isolated directly from this ecosystem, showing that this isolate persists in freshwater microcosms for at least one month. Paired transcriptomics and genome-scale fitness screening in creek water containing autochthonous microbiota identified E. coli genes required for persistence, including those involved in amino acid metabolism, nucleotide biosynthesis and biogenesis of curli. Isolate-resolved metatranscriptomics analysis supported these findings by revealing that many of these genes were highly expressed in situ . Deletion of curli structural genes resulted in reduced biofilm formation and a fitness defect specifically in the presence of freshwater microbiota, indicating that these structures promote environmental survival by improving E. coli competitiveness. Our study deepens our understanding of E. coli survival in waterways, while providing a broadly applicable framework for interrogating mechanisms of pathogen persistence in complex environments and communities.