Invasion dynamics of antimicrobial-resistant E. coli in river biofilms: impacts on the resistome, microbiomes, and horizontal gene transfer

River biofilms are exposed to invasion by antibiotic resistant bacteria (ARB) due to episodic or chronic exposure to wastewater, yet the ecological processes determining the fate of invaders and their resistance plasmids remain poorly understood. We experimentally exposed river-grown biofilms, originating from sites with contrasting microbial diversity and wastewater influence, to invasion by a genetically tagged ARB- E. coli carrying a transferable IncPα plasmid with the nptII resistance gene. Over two weeks, we quantified the dynamics of the invader and its plasmid, using qPCR and the plasmid-to-strain genome ratio in the biofilm as an indicator of horizontal gene transfer (HGT). We further characterized microbiomes and resistomes via 16S rRNA gene sequencing and metagenomics. Independent quantification methods provided highly consistent estimates of invasion dynamics: the invader established transiently across all biofilms, with ARB- E. coli abundance peaking within 48h and subsequently declining to near-background levels within 14 days. Plasmid-to-strain genome ratios decreased, indicating limited HGT and progressive plasmid loss. Wastewater-impacted biofilms showed slower declines, suggesting higher plasmid persistence potential in disturbed environments. The total community resistome exhibited pronounced but short-lived shifts whereas indigenous resistomes and microbiome composition remained stable. Yet, in one replicate from the wastewater-impacted site, specific indigenous ARGs of public health relevance increased, indicating that disturbance can promote localized ARG proliferation even without sustained invader establishment. Our results show that interactions between invaders and indigenous biofilm are dynamic and strongly shaped by community composition. This supports the One Health concept, highlighting how environmental context modulates AMR propagation risks in freshwater ecosystems.