Most strains of E.coli are harmless and live in the gut, but some, such as the O157 strain, cause food poisoning and are potentially fatal.

The University of Kiel research appears in Nature Immunology.

Clean fingers

The German researchers analysed the water people had used to wash their hands and other parts of their body, as well as skin scrapings from different body parts.

There were no living traces of certain strains of E.coli on the skin and in the water, even when they looked at conditions similar to those found when people get hot and sweaty.

Further laboratory tests enabled Dr Jens-Michael Schroeder and colleagues to pinpoint this down to psoriasin.

Psoriasin was originally discovered some years ago in people with the skin condition psoriasis, in larger quantities than normal.

It has also been found on the skin of newborn babies and might, therefore, protect the infant from infection during birth, the researchers believe.

The researchers say the findings explain why skin regions that are often exposed to high concentrations of E.coli, such as the skin around the anus, are rarely infected with this gut bug.

Bug busting

Higher levels of psoriasin were found on the hands, in the armpits and on the face.

Lower levels were found on the backs of the legs and arms.

"Psoriasin is probably key in the local innate [inborn] defence of healthy skin against the gut bacterium E.coli," said the authors.

Dr Mark Farrar, from the Skin Research Centre at Leeds University, said: "What's interesting is the mechanism by which it kills - using zinc.

"It's a novel mechanism."

But he said other characteristics of the skin were also important in preventing infection.

"The skin is very dry which is a hostile environment for bacteria. That could be a major factor as well."

Dr Farrar said that in the future it might be worth looking at other molecules that work in a similar way to psoriasin, to develop new antimicrobial products, particularly against more harmful or pathogenic strains of bacteria.

But he added: "It would take a while to develop something that could be used routinely."

He said scientists should monitor what effect psoriasin has on the naturally occurring skin bacteria.

"If they are resistant to killing [by psoriasin] you could look at what the mechanism of resistance is, which, in turn, could tell you whether pathogenic organisms are likely to develop resistance in the future," he said.