"Bacteria are by far the most active ice nuclei in nature," said Brent C. Christner, an assistant professor of biological sciences at Louisiana State University.

Christner and colleagues sampled snow from Antarctica, France, Montana and the Yukon and they report their findings in Friday's edition of the journal Science.

In some samples as much as 85 percent of the nuclei were bacteria, Christner said in a telephone interview. The bacteria were most common in France, followed by Montana and the Yukon, and was even present to a lesser degree in Antarctica.

The most common bacteria found was Pseudomonas syringae, which can cause disease in several types of plants including tomatoes and beans.

The study found it in 20 samples of snow from around the world and subsequent research has also found it in summer rainfall in Louisiana.

British scientists are ready to turn female bone marrow into sperm, cutting men out of the process of creating life.

The breakthrough paves the way for lesbian couples to have children that are biologically their own.

Gay men could follow suit by using the technique to make eggs from male bone marrow.

There are also fears that children born from artificial eggs and sperm will suffer severe health problems, like the mice in the Newcastle experiments.

Couples who have children from artificial sperm created from women would be able to have girls only. This is because the female sperm would lack the Y-chromosome needed for boys.

Robin Lovell-Badge, of the National Institute for Medical Research in London, said the creation of female sperm is at least a decade away.

Josephine Quintavalle, of Comment on Reproductive Ethics, a campaign group, said: "We are looking at absurd solutions to very obscure situations and not addressing the main issue. Nobody is interested in looking at what is causing infertility - social reasons such as obesity, smoking and age.