Abundances of tetracycline resistance genes and tetracycline antibiotics during anaerobic digestion of swine waste

Authors: COUCH, MELANIE - Western Kentucky University; Agga, Getahun; KASUMBA, JOHN - Western Kentucky University; Parekh, Rohan; Loughrin, John; CONTE, ERIC - Western Kentucky University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2018
Publication Date: 1/1/2019
Publication URL: https://handle.nal.usda.gov/10113/6471053
Citation: Couch, M., Agga, G.E., Kasumba, J., Parekh, R.R., Loughrin, J.H., Conte, E.D. 2019. Abundances of tetracycline resistance genes and tetracycline antibiotics during anaerobic digestion of swine waste. Journal of Environmental Quality. 48(1):171-178. https://doi.org/10.2134/jeq2018.09.0331.
DOI: https://doi.org/10.2134/jeq2018.09.0331

Interpretive Summary: Anaerobic digestion is a breakdown of organic matter by bacterial fermentation in the absence of oxygen. It is a commonly used animal waste management practice with the primary goal of biofuel production as a renewable energy source. Land application of effluent and sludge materials from animal waste management systems for crop production can result in the dissemination of manure-borne bacterial pathogens such as Salmonella, antibiotics, antibiotic resistant bacteria and their associated resistant genes. We evaluated the impact of anaerobic digestion of a swine waste for 100 days in three separate trials on the concentrations of tetracycline antibiotics and marker genes for tetracycline resistance in two identical digester tanks. In the end anaerobic digestion resulted in statistically significant but biologically small reductions in the concentrations of tetracycline resistance genes in the effluent, with no significant impact in the solid waste compared to the initial swine waste. Two tetracycline-antibiotics and one -metabolite were detected in the final digested waste product. Results from this study indicate that anaerobic digestion is unlikely to reduce the spread of antibiotic resistance genes in the environment.

Technical Abstract: Anaerobic digestion produces biogas and digested material from animal waste. Its effect on antibiotic resistance genes (ARGs) and antibiotics is not widely studied. We investigated the effect of anaerobic digestion on seven tetracycline resistance (Tetr) genes (tetA, B, G, M, O, Q and W), three tetracyclines (tetracycline, oxytetracycline and chlortetracycline), and two metabolites (4-epitetracycline and isochlortetracycline). Two identical 800-liter digesters were seeded with sludge from a swine waste lagoon and supplied antibiotic free feed in three separate 100 day trials. Tetr genes were measured by qPCR from total microbial community DNA extracted from initial swine waste lagoon, swine feed, corn mix, and digester samples (effluent, digestate liquid and sludge). Tetracyclines and their metabolites were extracted by solid phase extraction method and measured by HPLC-MS/MS. Concentrations of total bacteria and 6 tet genes (except tetM) in the effluent and digestate liquid samples showed significant reduction that ranged from 1 - 2.5 log10 reduction from their starting concentrations in the initial swine wastes. Concentrations of total bacteria and the tet genes in the sludge samples did not differ from their starting concentrations in the initial waste samples. Similar concentrations of total bacteria and tetracycline resistance genes could be detected from swine feed and corn samples. Up to 36 ppb (parts per billion; tetracycline), 112 ppb (chlortetracycline), 103 ppb (isochlortetracycline) and 30 ppb (4-epitetracycline) were detected; oxytetracycline was not detected. In conclusion, anaerobic digestion of animal waste has little to no effect in complete removal of bacteria, ARGs and antibiotics. Feeds used for animals and activation of anaerobic digestion can be a source of ARGs during the biodigestion process.