Bioavailability of Soil-Sorbed Tetracycline to <i>Escherichia coli</i> under Unsaturated Conditions

Increasing concentrations of anthropogenic antibiotics in soils are partly responsible for the proliferation of bacterial antibiotic resistance. However, little is known about how soil-sorbed antibiotics exert selective pressure on bacteria in unsaturated soils. This study investigated the bioavailability of tetracycline sorbed on three soils (Webster clay loam, Capac sandy clay loam, and Oshtemo loamy sand) to a fluorescent <i>Escherichia coli</i> bioreporter under unsaturated conditions using agar diffusion assay, microscopic visualization, and model simulation. Tetracycline sorbed on the soils could be desorbed and become bioavailable to the <i>E. coli</i> cells at matric water potentials of −2.95 to −13.75 kPa. Bright fluorescent rings were formed around the tetracycline-loaded soils on the unsaturated agar surfaces, likely due to radial diffusion of tetracycline desorbed from the soils, tetracycline uptake by the <i>E. coli</i> cells, and its inhibition on <i>E. coli</i> growth, which was supported by the model simulation. The bioavailability of soil-sorbed tetracycline was much higher for the Oshtemo soil, probably due to faster diffusion of tetracycline in coarse-textured soils. Decreased bioavailability of soil-sorbed tetracycline at lower soil water potential likely resulted from reduced tetracycline diffusion in soil pore water at smaller matric potential and/or suppressed tetracycline uptake by <i>E. coli</i> at lower osmotic potential. Therefore, soil-sorbed tetracycline could still exert selective pressure on the exposed bacteria, which was influenced by soil physical processes controlled by soil texture and soil water potential.