Transport of antibiotic-resistant bacteria and resistance-carrying plasmids through porous media

Microbial antibiotic resistance has emerged as not only a major medical problem but also as an environmental engineering challenge, with antibiotic resistance genetic elements as environmental pollutants. The transport characteristics of a tetracycline-resistant bacterium (B. cepacia) and plasmids carrying tetracycline-resistance genes were investigated using flow-through columns packed with porous media. Higher influent cell concentrations (1.1×10⁸ CFU mL⁻¹) resulted in higher breakthrough (C/C₀=0.596±0.055) than a solution with lower cell concentration (2.0×10⁶ CFU mL⁻¹, C/C₀=0.461±0.037). This decreased extent of filtration suggests fast initial cell deposition and strong subsequent blocking of binding sites, resulting in less-hindered microbial transport through the sandy medium. The addition of a bromide tracer (NaBr) prior to the plasmid solution resulted in DNA retardation and increased filtration in a zirconia-silica bead matrix. Apparently, Na⁺ binding to the beads decreased electrostatic repulsion between the negatively charged DNA and zirconia-silica surface. In contrast, plasmid breakthrough preceded that of the tracer when the plasmids were added first, possibly due to size exclusion chromatography coupled with stronger electrostatic repulsion. This implies that efforts to characterize the dynamics of resistance vector propagation in aquifers should consider the effect of groundwater chemistry and the surface characteristics of the porous media on vector transport.