University of Minnesota researchers say that the bacteria used to justify beach closings don’t always come from harmful sources. Mounting evidence suggests that Escherichia coli (E. coli) bacteria live as natural residents of 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.

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 1,000 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 1,000 milliliters) as previously permitted for swimming.