A bacterium that was thought to be a sign of sewage has now been found to thrive naturally in the wet sands that border temperate rivers and lakes. It could mean that freshwater swimming beaches are being closed down even when there is no sewage contamination.

E. coli is a common bacterium found in the intestines of birds and mammals. Some strains can cause food poisoning, but most are harmless. Because E. coli occurs in faeces – which are laden with other disease-causing bacteria and viruses – it is often used as an indicator for sewage pollution.

Richard Whitman of the US Geological Survey spent 10 years studying the shores of the Great Lakes, where beaches are sometimes closed due to high levels of E. coli.

Seagull droppings

He has found that concentrations of the bacterium often exceed 10,000 bugs per 100 millilitres of sand – enough when mixed with beach water to exceed the maximum safety level set by the Environmental Protection Agency.

But genetic tests showed that less than half of the sand bugs came from seagull faeces or the lake water. Whitman says this strongly suggests that the bugs occur naturally on the beaches.

He found that some bacteria even persist during the winter when the beaches are covered in snow and ice. “The bacteria are living there,” agrees Roger Fujioka, a microbiologist at the University of Hawaii.

Whitman claims this is the first time they have been shown to persist in beach sand. Any freshwater sandy shore could have high levels of E. coli, he says, and using the bug as an indicator for water quality may be flawed.