When clouds begin to rain and snow, the culprit is likely bacteria, according to two Montana State University professors and their colleagues.

Their findings, published today in the journal "Science," might hold a few secrets to understanding and reducing drought, said David Sands, a plant pathologist who has been studying the idea for more than 25 years.

"Bacteria have probably been around for a million years," Sands said. "They live on the surface of plants, and may occasionally cause plant disease. But their role in rain-making may be more important."

Although the implications might be enormous, he said, they have yet to be proven.

What the researchers do know, and outlined in their research, is that one particular bacteria n something called Psuedomonas syringae, a bacteria that first brought Sands to Montana n is at the heart of snow and ice crystals all over the world.

Science has long held that ice forms in clouds around a key ingredient called a nuclei. Most scientists believe that nuclei is a dust particle. But Sands and his fellow authors found that in a large number of cases the key ingredient is biological n the bacteria Psuedomonas syringae.

Biological nuclei are different from dust particles, Sands said, because they cause water to freeze at warmer temperatures. When the water freezes, it falls to the ground as rain or snow, taking the bacteria with it. Eventually, the bacteria lands on a plant, where it grows and multiplies n and soon is swept back into the atmosphere on the wind.

"We think if (the bacteria) couldn't cause ice to form, they couldn't get back down to the ground," Sands said. "As long as it rains, the bacteria grow."

And as long as the bacteria grow and float into the atmosphere, it rains.

But if the bacteria can't grow, Sands said, there's less snow and rain.

"So, if you get hungry and graze off all your plants," he said, "you (might) get drought."

Sands and his colleagues -Christine Foreman, an MSU professor of land resources and environmental sciences; Brent Christner, a microbiologist at Louisiana State University; and Cindy Morris, who runs a research lab in France - found the bacteria in the snow atop Montana ski resorts; in California, Australia and South Africa; and even in the ice in Antarctica.

"Bacteria are by far the most active ice nuclei in nature," Christner told the Associated Press in a telephone interview.

Christner said that in some of samples of snow taken from around the world, as much as 85 percent of the nuclei were bacteria.

Sands said the team pulled together its findings over two years, with the help of dozens of researchers, including meteorologists, cloud physicists, people who've been working on drought for years and microbiologists. They used DNA to identify the bacteria.

"These are people in little fields, and for them to pull out of the little fields and work on the big picture, it's beyond all of us," Sands said Thursday while sitting in his office on the second floor of MSU's plant science building.

Sands was not surprised by the findings. He had suspected that Psuedomonas syringae was a culprit since he arrived in Montana in 1976.

The bacteria was causing problems with wheat in Montana, and Sands had written a thesis on it. He soon realized that the bacteria could be found on previously clean wheat crops growing in fields far from other infected crops. And 1976 was a pretty rainy year.

A year later, he was looking to the clouds for an answer n flying high above the infected wheat fields near Ophein, Sands stuck his arm out the window and held a Petri dish to the wind.

"I even got a little frost bite," Sands said, rubbing the inside of his wrist.

On that Petri dish, Sands found Psuedomonas syringae.

Sands published a paper on the idea in 1982, but it garnered little attention. Now, because of the DNA tests, he and his team can show that the bacteria is not only on the plant and in the cloud n but at the heart of a snowflake.

"It's fun to see something come out after 25 years," Sands said, "particularly when we knew back then it was true."

More studies must be done because questions remain. Can other bacteria do the same thing? Does this one vary by region?

"I keep looking at that Bridger cloud and wondering how many bacteria are up there," Sands said, looking at the mountain range from the window in his office.

"I want people to be fascinated by the interconnection of things going on in the environment," Sands said. "It's all interconnected."

The Associated Press and Montana State University News Service contributed to this report.