The effect of woodchip bioreactors on microbial concentration in subsurface drainage water and the associated risk of antibiotic resistance dissemination

Abstract

Woodchip bioreactors (WBs) are a promising technology capable of decreasing nitrate concentration in subsurface (tile) drainage system (SDS) effluent. Although they are effective in removing nitrate, little is known regarding the potential impact on microbes and antimicrobial resistance (AMR) in subsurface drainage waters. Therefore, the main objective of this study was to measure how bioreactors influence microbial concentrations and AMR concentrations in tile drainage water with different flow conditions and microbial communities. A column study was conducted to simulate an existing, field denitrifying woodchip bioreactor. Four synthetic waters were spiked with different combinations of microbes isolated from the influent of a field-scale bioreactor and pumped through the columns during the experiment. Steady and wet-dry flow conditions were examined. The results of this work demonstrated the potential for denitrifying bioreactors to reduce E. coli concentrations within SDSs under different flow conditions, which expands the potential benefits of WBs. However, the results also illustrated that WBs have the potential to increase phenotypic antibiotic-resistant microbes in the subsurface drainage waters. Applying different microbial communities had significant effects on culturable microbial concentrations, as columns receiving more microbes with a greater diversity had a greater increase in culturable microbial concentrations. A statistically significant and consistent increase in phenotypic AMR concentrations was also found within woodchip bioreactors after assessing the sensitivity of recovered isolates to five different antibiotics. Additional laboratory and future in-field studies are warranted to improve the prediction of pathogen removal and AMR changes in tile drainage water, as well as to support the development of an effective design for microbial contaminant removal from waters passing through denitrifying bioreactors.