Nonmonotonic Effect of Montmorillonites on the Horizontal Transfer of Antibiotic Resistance Genes to Bacteria

Bacterial transformation is important to the transmission of antibiotic resistance genes (ARGs) in soils, and soil pore water is recognized as a hot spot for dissemination of ARGs. However, little is known about how concentrations and sizes of clay particles in soil pore water influence the horizontal transfer of ARGs. This study investigated the horizontal transfer of an antibiotic resistant plasmid (pUC19) into competent Escherichia coli cells with or without montmorillonites (MMTs, 0-2 g/L) with average sizes of 1568 nm (size-1500 fraction), 568 nm (size-500 fraction), and 300 nm (size-300 fraction). With increasing MMT concentrations, horizontal ARG transfer was initially enhanced and then became inhibited at transition concentrations of 0.100 g/L for the size-1500 fraction, 0.050 g/L for the size-500 fraction, and 0.025 g/L for the size-300 fraction. The smaller MMT size fractions had a stronger effect on horizontal ARG transfer. At lower MMT concentrations, horizontal ARG transfer was enhanced by an increased level of plasmid-cell contact and possible hole formation in the cell membrane, whereas at higher MMT concentrations, it was inhibited by binding of the plasmid to MMTs and its aggregation facilitated by released metal cations. Our results provided new insight into the influence of clay minerals on ARG transfer in soils, thus improving our understanding of the environmental fate of ARGs.