Tests used to determine if water at beaches is safe to swim in may not be accurate, according to a new study from the University at Buffalo and Mercyhurst University.

The problem: The most commonly used test fails to distinguish from toxic and more benign forms of contaminants. As a result, authorities sometimes close beaches when they don’t need to, or keep them open when they shouldn’t.

Health departments in Erie, Niagara and Chautauqua counties take water samples from 22 beaches – Erie on a daily basis at six major beaches – and test for fecal coliform and E. coli. Last year they closed 61 times because of high E. coli levels. But the testing doesn’t determine what strains of E. coli are found in the water, said Gerald Koudelka, University at Buffalo professor of biological sciences, who led the study, published in the journal Applied and Environmental Microbiology. He said the E. Coli strain with the Shiga toxin is what sickens people and animals.

Koudelka’s team took samples from Presque Isle State Park and Mill Creek Stream in Northern Pennsylvania. They added strains of E. coli with the Shiga toxin and strains that do not produce the toxin to see how they persisted with tiny single-cell bacteria grazing creatures called protists that already existed in the water samples. Researchers added more protists to samples in a separate experiment.

The study found that in both cases the strains with the Shiga toxin lasted longer in the water — up to 48 hours longer — than the strains without it.

Koudelka said this means that a beach may be opened because a test shows safe levels even though a majority of the bacteria measured could contain the Shiga toxin.

“That would not be good,” Koudelka said. “Using the test they do, nobody knows what fraction contains the Shiga toxin.”

The standard for fecal coliform is 200 colonies per 100 milliliters. Anything above that often results in a beach closing or an advisory.

“If just half of those colonies have the Shiga toxin, then there is a strong risk of human infection, but the beach could remain open because the levels are below the standard,” Koudelka said.

The other concern is that a beach could be closed because of high E. coli levels, but the water could be safe because none or very little of the bacteria has the Shiga toxin.

“This is the economic part of it,” he said. “It’s a problem because you might have a beach that’s closed for days even though it’s safe.”

The E. coli bacteria get in the water through various sources, including wildlife, sewer overflows and storm water runoff. Most E. coli strains aren’t dangerous and live happily in the bodies of humans and animals.

“Most of us don’t really want to swim in that, but that doesn’t mean the water is dangerous, it is just rather disgusting,” Koudelka said.

John Finster, a public health consultant for the Erie County Health Department, acknowledged the testing isn’t the most accurate. But it is easy to collect, sample and process, and results can come back within 22 hours at $16 per sample. Koudelka said it is much more expensive to conduct testing for different strains.

“We know it is a flawed indicator,” Finster said. “But it’s what we have.”