Published online 1 May 2008
Published in J Environ Qual 37:898-905 (2008)
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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a Oceanographic Center of Nova Southeastern Univ., 8000 N. Ocean Dr., Dania Beach, FL 33004
b Dep. of Infectious Diseases and Pathology, Univ. of Florida, Gainesville, FL 32611
c Dep. of Biological Sciences, Florida Atlantic Univ., Davie, FL 33314
d Coll. of Science and Mathematics, Fresno State Univ., Fresno, CA 93740
e contributed equally to this work
* Corresponding author ([email protected] ).
Received for publication June 14, 2007. Fecal bacteria have traditionally been used as indicator organisms to monitor the quality of recreational waters. Recent work has questioned the robustness of traditional indicators, particularly at seawater bathing beaches. For example, a study of Florida beaches found unexpectedly high abundances of Escherichia coli, fecal coliforms, and enterococci in beach sand. The aim of the present study was to explain these abundances by assessing the survival of E. coli and enterococci in beach sand relative to seawater. We used a combination of quantitative laboratory mesocosm experiments and field observations. Results suggested that E. coli and enteroccocci exhibited increased survivability and growth in sand relative to seawater. Because fecal bacteria are capable of replicating in sand, at least under controlled laboratory conditions, the results suggest that sand may be an important reservoir of metabolically active fecal organisms. Experiments with "natural" mesocosms (i.e., unsterilized sand or water rich in micropredators and native bacteria) failed to show the same increases in fecal indicators as was found in sterile sand. It is postulated that this was due to predation and competition with indigenous bacteria in these "natural" systems. Nonetheless, high populations of indicators were maintained and recovered from sand over the duration of the experiment as opposed to the die-off noted in water. Indicator bacteria may wash out of sand into shoreline waters during weather and tidal events, thereby decreasing the effectiveness of these indicators as predictors of health risk and complicating the interpretations for water quality managers.
Abbreviations: BHIB, brain-heart infusion broth • mEI agar, m-enterococcus agar plates with indoxyl-β-d-glucoside • PBS, phosphate-buffered saline • PSU, practical salinity units • USEPA, United States Environmental Protection Agency
Department of Civil and Environmental Engineering, Environmental and Water Studies, Stanford University, Stanford, California 94305-4020
Received 3 October 2008/ Accepted 6 January 2009
Enterococci are indicator bacteria used to assess the risk of acquiring enteric disease from swimming in marine waters. Previous work identified beach sands as reservoirs of enterococci which can be transported from the sand to the sea, where they may instigate beach advisories. The present study establishes that naturally occurring enterococci can replicate in beach sands under environmentally relevant conditions. In unseeded, nonsterile microcosm experiments, it was shown that intermittent wetting of sands by seawater, like that which would occur at the high tide line, stimulates the transient replication of enterococci at rates of 0.20 to 0.63 per day (equivalent to doubling times of 1.1 to 3.5 days). Replication was not observed in control microcosms that were not subjected to wetting. Enterococci were enumerated using both culture-dependent (membrane filtration and mEI media) and culture-independent (quantitative PCR [QPCR], 23S rRNA gene based) techniques, which allowed tracking of both culturable and total enterococcus populations. Inhibition of QPCR and DNA extraction efficiencies were accounted for in the interpretation of the QPCR results. The results provide evidence that enterococci may not be an appropriate indicator of enteric disease risk at recreational beaches subject to nonpoint sources of pollution.
* Corresponding author. Mailing address: Department of Civil and Environmental Engineering, Environmental and Water Studies, Stanford University, Stanford, CA 94305-4020. Phone: (650) 724-9128. Fax: (650) 725-3164. E-mail: [email protected]
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