Lake Superior Studies Point Out Problems With Using E. coli as an Indicator of Beach Contamination
June 12, 2007
- Michael J. Sadowsky, Professor
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- Randall E. Hicks, Professor
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In three recent peer-reviewed journal articles, researchers from the University of Minnesota provide evidence that the bacteria used to justify beach closings don't always come from harmful sources. Together, the papers add to mounting evidence that Escherichia coli (E. coli) bacteria live as natural residents in the beach environment.
"Our results indicate that E. coli comes from several sources and may survive and replicate in sand, sediment, soils, and algae in the water," said Michael Sadowsky, professor at the University of Minnesota, Twin Cities. "This could increase the bacteria counts found on beaches, especially if the counts are taken on windy days when the sediment and algae are churned up. Often it's assumed that E. coli found during beach monitoring is washed into the water from the land or comes from sewage overflows, and we've shown that's not always the case."
E. coli bacteria typically live in the intestines of warm-blooded animals (including humans and birds) and are used at most Great Lakes coastal beaches as an indicator for pollution and an increased risk for illness. While many strains are harmless, some cause gastrointestinal illnesses in humans. Symptoms include vomiting, diarrhea, or other more serious conditions. It's not clear, however, if the E. coli the researchers found living in and around the beach cause any harm to humans.
The papers were based on data collected from 2003 to 2005 in studies funded by the University of Minnesota Sea Grant Program.
"Understanding how E. coli survives and interacts in the environment can help change our interpretation of beach monitoring results," said Randall Hicks, professor at the University of Minnesota, Duluth. "It's all a question of risk . . . what's the relative risk of an indicator organism coming from a bird, versus a human, versus the sand."
Recently, health officials in Pennsylvania announced that beaches at Presque Isle State Park on Lake Erie will no longer close due to standard advisory levels of E. coli (235 colonies per 100 milliliters). The park revised its advisory system based on new health risk information and allows up to four times the amount of E. coli (1,000 colonies per 100 milliliters) as previously permitted for swimming.
The major findings of each paper are summarized below. Images of the beaches and algae examined, and copies of the research papers are available upon request from the researchers.
1) Ishii, S., W. B. Ksoll, R. E. Hicks, and M. J. Sadowsky. 2006. Presence and growth of naturalized Escherichia coli in temperate soils from Lake Superior watersheds. Applied and Environmental Microbiology 72:612-621.
The researchers found that strains of E. coli from soil near streams that enter Lake Superior were unique, suggesting that these strains became naturalized to the soil. They also observed seasonal variations in density of the soil E. coli. They found the greatest population densities in June to October and the lowest numbers during February to May. DNA fingerprint analyses indicates that the E. coli strains survive the winter in frozen soil, increase in numbers in the spring and summer, and are present over time. This is the first report of growth of naturalized E. coli in natural (nonsterile, nonamended) soils. The presence of these large populations of naturalized E. coli in northern soils, which erode into streams or lakes and get into waterways, may confound the use of this bacterium as an indicator of fecal contamination.
2) Ishii, S., D. L. Hansen, R. E. Hicks, and M. J. Sadowsky. 2007. Beach sand and sediments are temporal sinks and sources of Escherichia coli in Lake Superior. Environmental Science and Technology 41:2203-2209.
Researchers identified potential sources of E. coli in water, sediment, and beach sand at the Duluth Boat Club Beach, located on the Duluth-Superior Harbor, from spring to fall (April to October) in 2004 and 2005. E. coli counts increased during the summer and early fall (July to September).They found that in spring (April and May), E. coli likely originated from wastewater, while in early summer to fall (June to October), waterfowl became the primary source of E. coli at this beach. Less than 1 percent of the E. coli strains isolated from this beach were potentially pathogenic. These results indicate E. coli from humans and waterfowl can accumulate in beach sand and sediment, which serve as temporal sources and sinks of E. coli that contribute to the closure of this beach.
3) Ksoll, W. B. S. Ishii, M. J. Sadowsky, and R. E. Hicks. 2007. Presence and sources of fecal coliform bacteria in epilithic periphyton communities of Lake Superior. Applied and Environmental Microbiology 73(12): in press.
Fecal coliforms and E. coli were found in epilithic periphyton (i.e., the slimy algal layer covering rocks) at three beaches in Lake Superior and the Duluth-Superior Harbor, including the Duluth Boat Club Beach. The source of 2 percent to 44 percent of the E. coli isolated from periphyton during 2004 and 2005 could be identified, with waterfowl being the major source of E. coli in these periphyton communities. The primary potential sources for most E. coli (57 percent to 81 percent) in overlying waters at these beaches were waterfowl, periphyton, and wastewater. Although many of the E. coli isolated from periphyton originated from waterfowl and to a lesser extent from wastewater, other strains appeared to be unique to the periphyton and may have developed self-sustaining naturalized populations in these communities. E. coli attached to periphyton can detach and contribute to fecal coliform numbers measured in coastal waters. The presence, persistence, and possible naturalization of E. coli in periphyton communities further confounds the use of fecal coliforms as a reliable indicator of recent fecal contamination of recreational waters.
Dr. Michael Sadowsky is a McKnight Distinguished University Professor in the Department of Soil, Water, and Climate at the University of Minnesota, Twin Cities. Dr. Randall Hicks is a Professor in the Department of Biology at the University of Minnesota, Duluth. Satoshi Ishii is a Ph.D. degree candidate in the Soil Science Graduate Program at the University of Minnesota, Twin Cities. Winfried Ksoll received an M.S. degree in 2006 from the Water Resources Science Graduate Program at the University of Minnesota, Twin Cities. Dennis Hansen is an M.S. degree candidate in the Biology Graduate Program at the University of Minnesota, Duluth.
The researchers are completing examinations of potential sources of E. coli in water, sediment, and sand at two other beaches sampled during 2006 in the Duluth-Superior Harbor, Southworth Marsh, and the Blatnik Bridge Boat Landing. During summer 2007, Drs. Sadowsky and Hicks’ laboratories are starting a new two-year study to investigate the short-term changes (days to weeks) in the abundances of waterfowl and human-derived fecal bacteria at three Lake Superior beaches near Duluth, Minn.