Comparison of metals and tetracycline as selective agents for development of tetracycline resistant bacterial communities in agricultural soil

Environmental selection of antibiotic resistance may be caused by either antibiotic residues or coselecting agents. Using a strictly controlled experimental design, we compared the ability of metals (Cu or Zn) and tetracycline to (co)select for tetracycline resistance in bacterial communities. Soil microcosms were established by amending agricultural soil with known levels of Cu, Zn, or tetracycline known to represent commonly used metals and antibiotics for pig farming. Soil bacterial growth dynamics and bacterial community-level tetracycline resistance were determined using the [(3)H]leucine incorporation technique, whereas soil Cu, Zn, and tetracycline exposure were quantified by a panel of whole-cell bacterial bioreporters. Tetracycline resistance increased significantly in soils containing environmentally relevant levels of Cu (≥365 mg kg(-1)) and Zn (≥264 mg kg(-1)) but not in soil spiked with unrealistically high levels of tetracycline (up to 100 mg kg(-1)). These observations were consistent with bioreporter data showing that metals remained bioavailable, whereas tetracycline was only transiently bioavailable. Community-level tetracycline resistance was correlated to the initial toxicant-induced inhibition of bacterial growth. In conclusion, our study demonstrates that toxic metals in some cases may exert a stronger selection pressure for environmental selection of resistance to an antibiotic than the specific antibiotic itself.