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Amy R. Sapkota,^1,2 Frank C. Curriero,^1,3 Kristen E. Gibson,¹ and Kellogg J. Schwab¹

¹Department of Environmental Health Sciences, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA; ²Maryland Institute for Applied Environmental Health, College of Health and Human Performance, University of Maryland, College Park, Maryland, USA; ³Department of Biostatistics, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA

Abstract
Background: The nontherapeutic use of antibiotics in swine feed can select for antibiotic resistance in swine enteric bacteria. Leaking swine waste storage pits and the land-application of swine manure can result in the dispersion of resistant bacteria to water sources. However, there are few data comparing levels of resistant bacteria in swine manure–impacted water sources versus unaffected sources.

Objectives: The goal of this study was to analyze surface water and groundwater situated up and down gradient from a swine facility for antibiotic-resistant enterococci and other fecal indicators.

Methods: Surface water and groundwater samples (n = 28) were collected up and down gradient from a swine facility from 2002 to 2004. Fecal indicators were isolated by membrane filtration, and enterococci (n = 200) were tested for susceptibility to erythromycin, tetracycline, clindamycin, virginiamycin, and vancomycin.

Results: Median concentrations of enterococci, fecal coliforms, and Escherichia coli were 4- to 33-fold higher in down-gradient versus up-gradient surface water and groundwater. We observed higher minimal inhibitory concentrations for four antibiotics in enterococci isolated from down-gradient versus up-gradient surface water and groundwater. Elevated percentages of erythromycin- (p = 0.02) and tetracycline-resistant (p = 0.06) enterococci were detected in down-gradient surface waters, and higher percentages of tetracycline- (p = 0.07) and clindamycin-resistant (p < 0.001) enterococci were detected in down-gradient groundwater.

Conclusions: We detected elevated levels of fecal indicators and antibiotic-resistant enterococci in water sources situated down gradient from a swine facility compared with up-gradient sources. These findings provide additional evidence that water contaminated with swine manure could contribute to the spread of antibiotic resistance.

Key words: antibiotic resistance, CAFO, concentrated swine feeding operation, E. coli, enterococci, fecal coliforms, fecal indicators, groundwater, surface water. Environ Health Perspect 115:1040–1045 (2007) . doi:10.1289/ehp.9770 available via http://dx.doi.org/ [Online 22 March 2007]

Address correspondence to A.R. Sapkota, Maryland Institute for Applied Environmental Health, College of Health and Human Performance, University of Maryland, College Park, 2310 HHP Building, College Park, MD 20742 USA. Telephone: (301) 405-2438. Fax: (301) 405-8397. E-mail: amy@amysapkota.com

We thank the neighbors of the swine facility who kindly provided access to their properties, surface water sources, and groundwater sources.

This research was funded by The Johns Hopkins Center for a Livable Future, Bloomberg School of Public Health. A.R.S. was a Howard Hughes Medical Institute Predoctoral Fellow during this research.

The authors declare they have no competing financial interests.

Received 29 September 2006 ; accepted 22 March 2007.