Antimicrobial resistance dissemination via horizontal gene transfer is constrained in stratified waters

The study analyzes how antibiotic resistance genes (ARGs) spread in aquatic environments and finds that their movement between bacteria is strongly limited in stratified waters such as lakes and oceans. Using large metagenomic datasets, the researchers show that ARGs tend to remain within specific microbial lineages and water layers, rather than spreading freely between different layers of the water column. Water stratification—where warmer surface water sits above colder deeper water—acts as a physical and ecological barrier that restricts horizontal gene transfer and vertical mixing of resistance genes. The findings suggest that increasing and prolonged stratification due to climate change may further reduce connectivity between microbial communities, shaping how antimicrobial resistance evolves and spreads in aquatic ecosystems.