Tetracycline Transport Through Poultry Farm Soils and Aquifer Materials: Influence on Bacterial Tetracycline Resistance
Markus Hilpert, PhD; William P. Ball, PhD; Mandy Ward, PhD Objectives: The widespread use of antibiotics in concentrated animal feeding operations (CAFOs) introduces antibiotics, antibiotic-resistant bacteria (ARB), and antibiotic resistance genes (ARG) into local soils. The presence of these three components can lead to an amplification of antibiotic resistance genes in microbial communities, including those not directly associated with the production facilities (for example, in downstream water bodies). This, in turn, poses a public health threat, particularly when potential human pathogens become drug-resistant. While it is clear that extensive use of antibiotics results in the propagation of ARG, few studies have quantified the transport phenomena associated with the spread of antibiotic resistance—i.e., the sub-surface movement of antibiotics, ARG and ARB. We hypothesize that the sub-surface transport of dissolved antibiotic chemicals in ground water is the principal factor in the spread of antibiotic resistance genes (ARG). Methods: We propose to: (1) perform batch and column experiments that allow quantifying the transport properties (e.g., sorption) of the antibiotic tetracycline (Tc) in soil and aquifer samples collected from a CAFO; (2) perform column experiments to determine if Tc transport is correlated with the acquisition of antibiotic resistance in naïve bacteria (i.e. bacteria that have not previously been exposed to Tc); and (3) develop a numerical model to guide the experimental design and to develop hypotheses regarding the large-scale transport of antibiotics and antibiotic resistance. Results: Through collaboration with USDA, we used geophysical exploration methods to analyze the sub-surface of a CAFO on the Delmarva Peninsula for possible contamination by animal wastes. A map of apparent electrical conductivity indicates that the sub-surface has been affected by a waste storage facility and that groundwater flow and runoff have contributed to the spread of the contamination. Soil and aquifer samples have been collected from areas affected and not affected by the storage facility. Using microbiological plating experiments, we found that Tc-resistant bacteria are present in the sub-surface, with higher concentrations of culturable Tc-resistant bacteria in the top soil than in the underlying aquifer. Using PCR, we have so far identified one gene, tet(M), that contributes to Tc resistance. Molecular biology experiments based on shot-gun cloning are underway that will allow us to: (1) identify the most prevalent Tc resistance genes; and (2) evaluate whether the waste storage shed has resulted in amplification of Tc resistance genes in the subsurface. Conclusions: A CAFO on the Delmarva Peninsula releases substances to the sub-surface that increase the apparent electrical conductivity. Likely, this increase is due to dissolved ionic species in the animal waste. Tetracycline-resistant bacteria are present in the sub-surface of the CAFO. In future, we will evaluate whether the contamination by animal waste has caused an increase in Tc resistance. If this proves to be the case, one should consider more sustainable agricultural practices that minimize the accidental release of animal waste to the sub-surface and/or ensure that antibiotics in these wastes are decomposed when the waste is released to the sub-surface (either accidentally or intentionally as a fertilizer). Markus Hilpert was awarded a diploma in physics in 1993 and a PhD in civil engineering in 1997 from the University of Karlsruhe in Germany. Currently he is an assistant professor in the Department of Geography and Environmental Engineering at Johns Hopkins University. His research centers on flow and transport phenomena in sub-surface environments, with a recent emphasis on microbial processes. | 
