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The following article is interesting because it describes how microbes are high up in the sky riding air currents and winds to circle the earth, and eventually drop down somewhere. This is one way diseases can be spread from one part of the world to another. And the study looking at how antibiotic resistant bacteria are spread in the air from cattle feedlots has implications for how antibiotic resistance is spread. From Smithsonian:

Living Bacteria Are Riding Earth's Air Currents

Considering the prevailing winds, David J. Smith figured the air samples collected atop a dormant volcano in Oregon would be full of DNA signatures from dead microorganisms from Asia and the Pacific Ocean. He didn’t expect anything could survive the journey through the harsh upper atmosphere to the research station at the Mount Bachelor Observatory, at an elevation of 9,000 feet.

But when his team got to the lab with the samples, taken from two large dust plumes in the spring of 2011, they discovered a thriving bunch of hitchhikers. More than 27 percent of the bacterial samples and more than 47 percent of the fungal samples were still alive. Ultimately, the team detected about 2,100 species of microbes, including a type of Archea that had only previously been isolated off the coast of Japan. “In my mind, that was the smoking gun,“ Smith says. Asia, as he likes to say, had sneezed on North America.

 Microbes have been found in the skies since Darwin collected windswept dust aboard the H.M.S. Beagle 1,000 miles west of Africa in the 1830s. But technologies for DNA analysis, high-altitude collection and atmospheric modeling are giving scientists a new look at crowded life high above Earth. For instance, recent research suggests that microbes are hidden players in the atmosphere, making clouds, causing rain, spreading diseases between continents and maybe even changing climates.

"I regard the atmosphere as a highway, in the most literal sense of the term," Smith says. "It enables the exchange of microorganisms between ecosystems thousands of miles apart, and to me that’s a more profound ecological consequence we still have not fully wrapped our heads around."

Airborne microbes potentially have huge impacts on our planet. Some scientists attribute a 2001 foot-and-mouth outbreak in Britain to a giant storm in north Africa that carried dust and possibly spores of the animal disease thousands of miles north only a week before the first reported cases. Bluetongue virus, which infects domestic and wild animals, was once present only in Africa. But it's found now in Great Britain, likely the result of the prevailing winds.

In west Texas, researchers from Texas Tech University collected air samples upwind and downwind of ten cattle feedlots. Antibiotic resistant microbes were 4,000 percent more prevalent in the downwind samples. .... What's clear is there are far more viable microbes in far more inhospitable places than scientists expected.