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Lake Superior Studies Point Out Problems With Using E. coli as an Indicator of Beach Contamination

Water not recommended sign

In three recent peer-reviewed journal articles, researchers funded by Minnesota Sea Grant 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 with the University of Minnesota Department of Soil, Water, and Climate. "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.

Left to Right: Michael Sadowsky, Randall Hicks

The papers are based on data collected from 2003 to 2005.

"Understanding how E. coli survives and interacts in the environment can help change our interpretation of beach monitoring results," said Randall Hicks, professor with the University of Minnesota Duluth Department of Biology. "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."

Health officials in Pennsylvania announced this spring 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.

To order copies of the journal articles see the Journal Reprint section and select among JR 533, 537, and 539.


By Marie Zhuikov
August 2007

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