When an antibiotic is consumed, researchers have learned that up to 90 percent passes through a body without metabolizing. This means the drugs can leave the body almost intact through normal bodily functions.
In the case of agricultural areas, excreted antibiotics can then enter stream and river environments through a variety of ways, including discharges from animal feeding operations, fish hatcheries, and nonpoint sources such as the flow from fields where manure or biosolids have been applied. Water filtered through wastewater treatment plants may also contain used antibiotics.
Consequently, these discharges become "potential sources of antibiotic resistance genes," says Amy Pruden, a National Science Foundation CAREER Award recipient, and an assistant professor of civil and environmental engineering at Virginia Tech.
"The presence of antibiotics, even at sub-inhibitory concentrations, can stimulate bacterial metabolism and thus contribute to the selection and maintenance of antibiotic resistance genes," Pruden explains. "Once they are present in rivers, antibiotic resistance genes are capable of being transferred among bacteria, including pathogens, through horizontal gene transfer."
The World Health Organization and the Center for Disease Control recognize antibiotic resistance "as a critical health challenge of our time," Pruden writes in a paper published in a 2010 issue of Environmental Science and Technology.
Pruden says reducing the spread of antibiotic resistance is a critical measure needed to prolong the effectiveness of currently available antibiotics. This is important since "new drug discovery can no longer keep pace with emerging antibiotic-resistant infections," Pruden says.
Pruden who has developed the concept of antibiotic resistance genes as environmental pollutants has an international reputation in applied microbial ecology, environmental remediation, and environmental reservoirs of antimicrobial resistance.
In her work outlined in the Environmental Science and Technology article, she and her co-authors, H. Storteboom, M. Arabi and J.G. Davis, all of Colorado State University, and B. Crimi of Delft University in The Netherlands, identified specific patterns of antibiotic resistance gene occurrence in a Colorado watershed. Identification of these patterns represents a major step in being able to discriminate between agricultural and wastewater treatment plant sources of these genes in river environments.
They assert that such unique patterns of antibiotic resistance gene occurrence represent promising molecular signatures that may then be used as tracers of specific manmade sources.
In their study they identified three wastewater treatment plant sites, six animal feeding operation locations, and three additional locations along a pristine region of the Poudre River, in an upstream section located in the Rocky Mountains. They compared the frequency of detection of 11 sulfonamide and tetracycline antibiotic resistance genes.
Their findings showed detection of one particular antibiotic resistance gene in 100 percent of the treatment plant and animal feeding operations, but only once in the clean section of the Poudre River.
As they are able to differentiate between human and animal sources of the antibiotic resistance genes, Pruden and her colleagues believe they can "shed light on areas where intervention can be most effective in helping to reduce the spread of these contaminants through environmental matrixes such as soils, groundwater, surface water and sediments.
"This study advances the recognition of antibiotic resistance genes as sources to impacted environments, taking an important step in the identification of the dominant processes of the spreading and transport of antibiotic resistance genes."
The Colorado Water Resources Research Institute and a USDA Agricultural Experiment Station provided funding for this study in addition to Pruden's NSF award.
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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Virginia Tech, via EurekAlert!, a service of AAAS.
- Heather Storteboom, Mazdak Arabi, Jessica G. Davis, Barbara Crimi, Amy Pruden. Tracking Antibiotic Resistance Genes in the South Platte River Basin Using Molecular Signatures of Urban, Agricultural, And Pristine Sources. Environmental Science & Technology, 2010; 44 (19): 7397 DOI: 10.1021/es101657s
A novel approach utilizing antibiotic-resistance-gene (ARG) molecular signatures was applied to track the sources of ARGs at sites along the Cache la Poudre (Poudre) and South Platte Rivers in Colorado. Two lines of evidence were employed: (1) detection frequencies of 2 sulfonamide and 11 tetracycline ARGs and (2) tet(W) phylotype and phylogenetic analysis. A GIS database indicating the locations of wastewater treatment plants (WWTPs) and animal feeding operations (AFOs) in the watershed was also constructed to assess congruence of the surrounding landscape with the putative sources identified by ARG molecular signatures. Discriminant analysis was performed on detection frequencies of tetARG groups that were previously identified to be associated with either WWTPs or AFOs. All but one (South Platte River-3, just downstream from the confluence with the Poudre River) of the eight sites were classified as primarily WWTP-influenced based on discriminant analysis of ARG detection frequencies. tet(W) phylotype analysis also aligned South Platte River-3 with putative AFO sources, while phylogenetic analysis indicated that it was not significantly different from the AFOs or WWTPs investigated. South Platte River-3 is situated in an intense agricultural area, but the upstream portion of the South Platte River receives substantial loading from metropolitan Denver. By contrast, tet(W) phylotype and phylogenetics of site Poudre River-4, located 4 km downstream of a WWTP, was also characterized and found to be significantly different from the AFO lagoons (p < 0.05), as expected. In general, a good correspondence was found between classification of the impacted river sites and the surrounding landscape. While the overall approach could be extended to other watersheds, the general findings indicate that transport of ARGs from specific sources is likely the dominant mechanism for ARG proliferation in this riverine environment relative to selection of ARGs among native bacteria by antibiotics and other pollutants.