Microbial community diversity predicts invasion resistance of freshwater biofilms against antibiotic-resistant bacteria

Rivers receive continuous inputs of antibiotic-resistant bacteria (ARB) from wastewater, agriculture, and other anthropogenic sources, yet it remains unclear whether the recipient ecological component and its microbial communities determine whether introduced ARB establish or disappear. Ecological invasion theory predicts that invasion success depends on biodiversity, community stability, and occupation of ecological niche space, but these mechanisms have rarely been evaluated together in natural microbial communities. Here, we challenged river biofilms collected from 20 sites in 12 European rivers across six countries with a model antibiotic-resistant Escherichia coli carrying a conjugative IncP-1 plasmid The invasion assays were carried out under standardized laboratory flume conditions. River biofilms differed markedly in their permissiveness to invasion despite identical invasion conditions. Higher bacterial diversity consistently accelerated invader loss rates, whereas communities containing more abundant and diverse close phylogenetic neighbours of the invader exhibited stronger exclusion during early biofilm establishment. Diversity loss during transition into the experimental system emerged as the strongest explanatory variable of invasion resistance prior to biofilm maturation, whereas Shannon diversity became the dominant predictor in mature communities. Integrating these complementary ecological dimensions substantially improved explanatory prediction of ARB persistence compared with individual predictors alone. Particularly invasion-resistant biofilms also exhibited distinct ecological community composition consistent with mature, structurally complex microbial assemblages. Together, our findings demonstrate that the establishment of ARB in the environment is not stochastic but can be predicted from measurable ecological properties of recipient microbiomes, highlighting microbial biodiversity and community organization as natural barriers to antimicrobial resistance dissemination.