Plasmid-mediated horizontal transfer of ARGs in Daphnia magna-associated microbiota: The role of tetracycline exposure and underlying mechanisms

The spread of antibiotic resistance genes (ARGs), particularly through plasmid-mediated horizontal gene transfer, has emerged as a significant health concern. Tetracycline (TET) is among the most prevalent antibiotics in the aquatic environment. As a key crustacean zooplankton, Daphnia magna may ingest antibiotic-resistant microorganisms and promote the spread of ARGs under TET pressure. Herein, intragenus and intergenus conjugative transfer experiments were established between a donor strain, Escherichia coli MG1655 (carrying the multidrug-resistant RP4 plasmid), and D. magna -associated microbiota. Results demonstrated that TET exposure enhanced the horizontal transfer of RP4 to both recipient bacteria ( E. coli HB101) and D. magna -associated microbiota. The underlying mechanism involved the overproduction of reactive oxygen species, increased secretion of extracellular polymeric substances, enhanced membrane permeability, and improved energy metabolism. Furthermore, the RP4 plasmid was transferred to D. magna -associated microbiota via ingestion, and the predominant taxa within D. magna -associated microbiota served as a transconjugal pool for the plasmid. Specifically, TET exposure promoted the enrichment of potential pathogen taxa in D. magna -associated microbiota. Given the pivotal role of D. magna in freshwater food chains, the trophic transfer of ARGs induced by TET residues in aquatic environments warrants serious consideration.