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Antibiotics in Groundwater Change Bacterial Ecology

White tanks used to hold the tracer injection solutions with tubing going to injection wells
The study was conducted within the zone of the historic wastewater-contamination plume at the Cape Cod Toxic Substances Hydrology Research Site, Massachusetts. Sulfamethoxazole and bromide (contained in the plastic drums on the far right) were repeatedly injected into the row wells (in the center of the photo) for 30 days. The sulfamethoxazole traveled through the subsurface to the observation wells on the far left. Photo credit: Denis LeBlanc, USGS.

Close up of a filter chamber used to hold bacteria colonies for testing
Bacteria contained in filter chambers placed in downgradient observation wells were exposed to sulfamethoxazole for 30 days as it traveled past the wells. Photo credit: Ron Harvey, USGS.

U.S. Geological Survey (USGS) scientists conducted a unique study which demonstrated that the ecology of natural groundwater bacteria changed after exposure of the bacteria to the antibiotic sulfamethoxazole (SMX). Because SMX has been found in environmental waters in previous scientific studies, USGS scientists conducted a field test to determine the effects of SMX on natural groundwater bacteria. Natural communities of groundwater bacteria were exposed to concentrations of SMX 2 to 3 orders of magnitude less than concentrations used in the treatment of antibiotic-resistant bacteria in humans. Following exposure, natural bacterial communities not previously exposed to SMX or other contaminants showed more mortality and more change in SMX resistance than did communities previously exposed to contamination. Although sharing of resistance genes among bacteria has been a focus of antibiotic-resistance studies for human health, there was no evidence of the genes commonly responsible for shared SMX resistance in this study. Instead, exposure to SMX changed the populations of bacteria that made up the community and the more antibiotic-resistant community members were left. This study indicates that SMX contamination may change the nature of bacterial flora in the environment and supports previous laboratory work indicating that natural bacterial processes and ecological functions such as denitrification may be affected by environmentally occurring SMX.

This study was funded by the USGS Toxic Substances Hydrology Program.


Haack, S.K., Metge, D.W., Fogarty, L.R., Meyer, M.T., Barber, L.B., Harvey, R.W., LeBlanc, D.R., and Kolpin, D.W., 2012, Effects on groundwater microbial communities of an engineered 30-day in situ exposure to the antibiotic sulfamethoxazole: Environmental Science and Technology, v. 46, no. 14, p. 7478-7486, doi:10.1021/es3009776.

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