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Bioavailability of Soil-Sorbed Tetracycline to Escherichia coli under Unsaturated Conditions
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posted on 2017-05-19, 16:18 authored by Zeyou Chen, Wei Zhang, Gang Wang, Yingjie Zhang, Yanzheng Gao, Stephen A. Boyd, Brian J. Teppen, James M. Tiedje, Dongqiang Zhu, Hui LiIncreasing
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 Escherichia coli 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 E. coli 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 E. coli cells, and its inhibition on E. coli 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 E. coli 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.