Fate and transport of persistent tetracycline residues in the environment
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Tetracycline antibiotics from human and veterinary use can enter the environment through the land-application of sewage sludge and/or animal manure that are used as soil conditioners. There has been an increased concern that animal diets containing tetracyclines may lead to the emergence of drug resistance among pathogenic microbes due to prolonged exposure to low levels of these persistent antibiotics. Research studies in this dissertation are aimed at: (1) detection of tetracycline antibiotics in manure fertilized soil and wastewater, (2) investigation of the factors that affect mobility of these residues in soil, and (3) elucidation of the mechanisms of interaction of tetracycline with clay and organic matter. ELISA concentrations showed that oxytetracycline and chlortetracycline persist in the soil fertilized with contaminated manure for several months. Liquid chromatography coupled with mass spectrometry (LC-MS) confirmed the presence of tetracycline compounds in wastewater effluent samples showing incomplete degradation which may result due to development of resistance in the mixed-culture microbes that are used in the biodegradation facility at a local wastewater treatment plant. Several mechanisms of non-covalent interactions of oxytetracycline and tetracycline in clay and clay associated humic are proposed based on the adsorption isotherms and the results from X-ray diffraction (XRD), Fourier transform infrared (FT-IR), attenuated total reflectance (ATR) FT-IR, one-dimensional nuclear magentic resonance (NMR) analyses. Two-dimensional NMR and mass spectrometry (MS) analyses provide evidence of the formation of novel covalent adducts of tetracycline with the oligomers of model humus constituent (syringic) and soil organic matter. Our results indicate that initially the adsorption of tetracycline into soil occurs via non-covalent interactions. Longer these residues remain sequestered in soil results in their slow chemical incorporation into humus via covalent bonding. Because phenolic diketone moiety is the principal active center in tetracycline group of antibiotics, the involvement of one of the phenolic oxygens attached to tetracycline aromatic ring in covalent bonding with humic substances can diminish its mobility, bioavailability, and bioactivity.
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