Bacterial Shiga toxin may be a blind spot in beach water testing

By on July 3, 2013
A Lake Michigan beach in Chicago (Credit: Chris Hamby, via Flickr)

Not all strains of E. coli are created equal.

At least that is the consensus from researchers at the University at Buffalo and Mercyhurst University, who collaborated on a study examining the impacts that various strains of E. coli have on their aquatic predators and on humans.

It has long been known that E. coli bacteria can be harmful to humans. The bacteria, which is commonly found in the lower intestines of warm-blooded animals, is often transferred through contaminated food products, but also can be potentially dangerous when present in various waterways frequented by humans.

Led by University at Buffalo’s Professor of Biological Sciences Gerald Koudelka, researchers began investigating two years ago how various kinds of E. coli react to predators and impact humans.

The study focused on two main types of E. coli: Shiga toxin-encoding E. coli (STEC) and non-Shiga encoding E. coli.

The STEC strain produces Shiga toxin, which is a compound that is poisonous to humans. The non-Shiga encoding E. coli don’t produce this poison and are generally less dangerous.

To test the impact that the toxins might have on protists, which are single-cellular organisms that feed on and help terminate E. coli, researchers collected water samples from Presque Isle State Park and Mill Creek Stream in northern Pennsylvania.

The researchers then placed three strains of the STEC strain into the samples along with three toxin-free strains to determine how the Shiga toxins might affect the protists.

Using equipment from Applied Biosystems, the researchers performed quantitative PCR testing to quantify the number of bacteria colony units forming for each of the different strains.

The results were conclusive: The E. coli that produced the Shiga toxins were able to fend off attack by grazing protists with much more efficiency than the E. coli that did not produce the toxins.

The protist Tetrahymena, shown in this photo illustration, is a predator of E. Coli bacteria. The Shiga toxin may provide some strains with extra defense. (Credit: University of Buffalo)

The protist Tetrahymena, shown in this photo illustration, is a predator of E. Coli bacteria. The Shiga toxin may provide some strains with extra defense. (Credit: University of Buffalo)

Over the course of 24 hours, the STEC populations decreased by 1.4-fold, while the non-toxic populations decreased by 2.5-fold. The tests also showed that the strain of E. coli that produced the most toxins lasted the longest in the water against predation.

Understanding that the bacteria strains that are the most harmful to humans are also the most resistant to predators could have wide-ranging impacts in improving the safeguards for protecting human health in recreational waterways.

“The question is: What is really harmful to humans?” Koudelka said. “Having human waste bacteria in the water is disgusting, but it may not be that harmful. So what you’re really worried about are the bacteria that can make you sick,”

The results of the study could lead to more specific water quality tests that single out strains of E. coli that are most harmful to humans. The current fecal indicator bacteria test mandated by the U.S. Environmental Protection Agency for assessing the water quality of recreational waters has no requirement to test for different strains of the bacteria, which leaves the potential for hazardous waters to go unchecked.

Because current mandates only put restrictions on the total amount of bacteria that can be present in the water rather than the type of bacteria, it is possible to have a beach remain open while harboring potentially harmful levels of toxin-producing E. coli and closed while containing higher levels of harmless non-toxic bacteria.

“The current beach water quality tests don’t actually look at toxins, they don’t separate or segregate out those bacteria—they lump them all together. And that can be an economic issue,” Koudelka said. “The issue is, you can have a situation where virtually all of those bacteria are things that will cause you harm, and you could [have a beach] be open, you wouldn’t know that.”

Although more sophisticated tests that identify different strains of E. coli are permissible in monitoring water quality for federal standards, researchers are required to present data corroborating their measurements. With the EPA’s standards frequently getting revised, uniform testing could be in the works.

“I think our study really points out the fact that we might need to have more finely grained viewpoints on [testing], and also, I think indicates that there are blanket tests that miss things that are already there in the water,” Koudelka said.

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