Antibiotic Pollution May Increase Groundwater Nitrate

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SIDE EFFECTS Antibiotics, such as sulfamethoxazole, could kill beneficial bacteria that clean up groundwater.

Decades after spawning a health care revolution, antibiotics are now common pollutants. Scientists' biggest concern about these emerging contaminants is that they promote the spread of resistance. But new research suggests they also harm the microbes that cleanse groundwater of dangerous compounds, particularly nitrates (Environ. Sci. Technol., DOI: 10.1021/es103605e).

High nitrate levels in drinking water can cause methemoglobinemia, a disease that decreases the blood's oxygen carrying capacity. Naturally-occurring bacteria in groundwater, such as Pseudomonas putida, can remove nitrates by reducing them to nitrogen gas.

In Cape Cod, Mass., very high nitrate levels co-occur in groundwater with one of the most common antibiotics in the clinical arsenal: sulfamethoxazole (SMX). For nearly a decade, microbiologist Ronald Harvey and colleagues from the U.S. Geological Survey have tracked SMX and other groundwater pollutants at an aquifer that originates at the Otis Air National Guard Base, a heavily polluted site on Cape Cod. Other researchers had shown that high doses of SMX can interfere with bacterial nitrate reduction. But no one knew if a similar response might occur at environmentally-relevant concentrations.

To answer that question, Harvey's team first cultured bacteria from a non-contaminated portion of the aquifer. Next, they added nitrate to the cultures at levels measured in the environment, along with SMX at doses ranging from 0.005 to 2,000 µM. Bacterial growth rates dropped at all doses. At the environmentally relevant concentration of 0.005 µM SMX, the amount of total nitrate that the bacteria removed from the culture fell by nearly half. "We're demonstrating a clear biological effect," Harvey says. "And we're showing that in the same bacteria that live in this particular aquifer."

Harvey says that the next step would be to study the bacteria's response to SMX exposure in the aquifer itself.

USGS research hydrologist Dana Kolpin, who didn't participate in this study, thinks the results suggest that SMX contamination might account at least in part for high nitrate levels in the aquifer. "We can't jump to that conclusion yet," he says. "But the data suggest that's a hypothesis worth pursuing."

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