Campbell, B. M. 2008. Characterization of Tetracycline efflux genes in soil bacteria and an analysis of environmental factors controlling their expression. MS Thesis, Michigan State University, East Lansing, MI, USA.

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Un-altered tetracycline (TC) residues disseminate to soil environments through the practice of applying manure to cropland as fertilizer from animals receiving sub-therapeutic doses of tetracycline for growth promotion purposes. Such practice has created concern that these residues are enhancing the development and transfer of TC resistance in soil environments. However, multiple knowledge gaps exist that need to be examined to address such concerns. First, few investigators have studied tetracycline resistance in soils by molecular methods. Second, it is unknown if TC residues present in the soil environment are bioavailable to microbial cells to exert a selective pressure.

Two separate sets of experiments were conducted to bridge these gaps. TC resistance was examined in an agricultural soil, collected one week after manure application by both culture dependent and independent methods. Novel findings include the discovery of tet gene variants tet (30), and tet (31) in soil, and the first observations of tet (A) in Microbacterium spp., tet (Y) in Pseudomonas spp., tet © and tet (31) in Stenotrophomonas spp., and tet (A) and tet © in Thermomonas spp.

In the second set of experiments bioavailability of cation chelated TC in solution was examined with a tetracycline bioreporter containing a gfp gene fused to a tetracycline inducible promoter. Both TC-Mg 2+ and TC-Ca2+ chelation significantly reduced TC efflux gene expression whereas monovalent cations (Na+ and K + ) had no effect.

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