Germs Can Travel the World by "Hitchhiking" on Dust

Sara B. McPherson
for National Geographic News
December 20, 2007
Microbes can traverse oceans and continents by hitching rides on minute dust particles, scientists report.

By analyzing dust samples originally collected by naturalist Charles Darwin, researchers have determined that bacteria and tiny fungi originating in the western Sahara desert are also found as far away as North America and the Caribbean.

Swirling windstorms in northern Chad sweep up sands from the Sahara, carrying the smallest particles into the troposphere, 1 to 6 miles (2 to 10 kilometers) above Earth's surface.

The fine grains are then blown across the Atlantic Ocean, settling nearly 6,200 miles (10,000 kilometers) away.

Anna Gorbushina and William Broughton at Switzerland's University of Geneva had long been interested in what the dust carried on its intercontinental travels.

"We've known about this phenomenon for centuries," Broughton said.

But not enough of the dust was available for study until a colleague suggested testing Darwin's samples, he explained.

Their team's findings appear in the current issue of Environmental Microbiology.

Microbial Hitchhikers

In the early 1800s, Darwin and others collected some of this aerial dust from Barbados and while at sea in the Atlantic Ocean.

Using only the most abundant samples, the authors employed geochemical analyses to confirm that Darwin's collections contained dust originating in West Africa.

In addition the team also found that the dust particles were carrying microbial hitchhikers.

"We found several species of bacteria and fungi … on the small particles," Broughton said.

The team was even able to cultivate and grow several of the microbes, proving that these microorganisms can remain viable for centuries.

Citing the harsh elements the microbes must endure, including severe weather and ultraviolet radiation, Broughton said he believes that only certain microbes can survive the trip.

"It's likely that only those bacteria and fungi that have protective structures, like spores, can survive the travels," he explained.

Microbes that dwell on arid rock and soil are better able to withstand the dryness and radiation that accompany intercontinental travel, he added. As a result, deserts are the only terrestrial environments where these traveling microbes can be found in abundance.

(Read related story: "Humans Wear Diverse 'Wardrobe' of Skin Microbes, Study Finds" [February 6, 2007].)

A Pathway for Disease?

The newly studied phenomenon begs the question: If bacteria and fungi can be carried on the wind from country to country, could disease travel the same path?

"Most normal human pathogens are not resistant enough [to survive the trip]," Broughton said.

"Unless there's some great change in the western Sahara, disease [traveling in this manner] doesn't pose a health threat."

While the spread of disease may not be an issue, scientists believe that the study confirms an important theory with global implications.

"We know very little about the longevity of microbes and their ability to tolerate long periods of stress," said Kenneth Timmis, head of the Environmental Microbiology Laboratory at the Helmholtz Centre for Infection Research.

The new study not only aids in scientists' understanding of how microbes survive, it also provides important evidence of how major atmospheric pathways influence the spread of microbial life on the planet, he said.

"This is a timely reminder that almost all parts of the biosphere connect," Timmis observed.

Broughton agreed: "This study means that the world is not an isolated place. In a microbiological sense, we are all part of one planet."

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