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Antibiotics in Groundwater Affect Natural Bacteria

A USGS scientist prepares a sample within a glove box
A USGS scientist prepares a sample to test the effect of antibiotics on denitrifying bacteria within a glove box. A glove box allows scientists to work with samples in an anaerobic (no oxygen) atmosphere, the conditions under which denitrification occurs. Photo credit: Jennifer C. Underwood, USGS.

U.S. Geological Survey (USGS) scientists have documented adverse health effects on populations of native soil bacteria exposed to levels of the antibiotic (SMX) below those used to treat diseases (subtherapeutic). Because SMX has been found in environmental waters by many previous studies, the scientists conducted laboratory exposure experiments to determine the effect of SMX on native bacteria. They found that SMX concentrations previously found in aquatic environments (approximately 1 microgram per liter [μg/L]) delayed the start of cell growth, limited denitrification (a critical component of global nitrogen cycles), and altered bacterial community composition. Adverse effects on soil bacteria in tests conducted at higher antibiotic levels (250 μg/L or greater), but still much less than levels prescribed to treat (therapeutic) or protect against (prophylactic) diseases, were even more pronounced.

Sulfamethoxazole in the Environment

SMX is a sulfonamide antibiotic that is commonly used to treat a variety of bacterial infections. Previous studies have documented that SMX is a contaminant in both U. S. streams and groundwater, and that wastewater treatment plants and septic tanks are sources of antibiotics to the environment. Although SMX has been shown to degrade in streams, groundwater may be particularly vulnerable to the persistence of SMX. The natural attenuation of SMX in groundwater appears to be constrained due to the absence of photodegradation by sunlight when compared to natural attenuation of SMX in streams.

Antibiotic Resistance Concerns

Because of well-documented cases of antibiotic resistance in hospitals and other settings, many scientists are studying the potential effects of antibiotics in the environment and the potential spread of antibiotic resistant bacteria. Other potential adverse effects on native bacterial populations in the environment remain less commonly studied.

Nitrogen Cycle May Be Affected

It is clear from this study that inhibition of critical ecological functions such as denitrification can occur when native denitrifying bacteria are exposed to environmentally relevant doses of SMX. Denitrification is the microbially facilitated process that converts nitrate to nitrogen gas, the main component of the earth’s atmosphere. Denitrification also plays a key role in soil fertility as well as in natural assimilation of nitrate pollution. The implication that these processes may be taking place in nature where wastewaters are entering water resources is of global importance to environmental and, by association, human health. Too much nitrogen, as nitrate, in groundwater used for drinking water can be harmful to infants.

Diagram of the nitrogen cycle with were antibiotics impact the cycle
This diagram of the nitrogen cycle shows were in the cycle antibiotics could impact the ability of denitrifying bacteria to process nitrates and nitrites in groundwater. The diagram is a modified version of figure 9 from USGS SIR 2004-5144, page 16.

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


Underwood, J.C., Harvey, R.W., Metge, D.W., Repert, D.A., Baumgartner, L.K., Smith, R.L., Roane, T.M., and Barber, L.B., 2011, Effects of the antimicrobial sulfamethoxazole on groundwater bacterial enrichment: Environmental Science and Technology, v. 45, no. 7, p. 3096-3101, doi:10.1021/es103605e.

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