Scientists have discovered for the first time that microbes that cause disease in humans can travel thousands of miles on high-altitude winds.
The winds studied carried a surprising diversity of bacteria and fungi, including known pathogens and some with genes for resistance to multiple antibiotics. Some of the microbes appeared to be alive – in other words, they had survived the long journey and were able to reproduce.
The researchers said this intercontinental transport route was unlikely to cause disease in humans because the concentration of microbes was low. However, they said it was a concern that microbes could be seeded into new environments and that genes for antibiotic resistance could travel this way.
The study found that the microbes rode a 1,200-mile (2,000 km) ride on dust particles blown from agricultural fields in northeastern China to Japan. Similar wind patterns exist around the world. More than 300 species of bacteria and about 260 species of fungi were found in samples collected over Tokyo. It is thought that other microbes are also present that are not yet known to science.
Prof Xavier Rodó from the Barcelona Institute for Global Health, who led the research team, said: “About 30-40% of the microbes were potentially pathogenic species, either well-recognised human pathogens or opportunistic pathogens (those affecting people with weakened immune systems).”
The study “is a warning that we need to change our view of the air,” Rodó added, particularly the idea that air at higher altitudes is almost sterile.
“We should use the new methods to take samples and see what is there. These bacteria and fungi are able to withstand very high and strong conditions in the environment.”
The analysis, published in the journal Proceedings of the National Academy of Sciencesused an airplane to collect 22 dust samples from the air between 0.6 and 1.9 miles above Japan. The higher samples were above the planetary boundary layer (PBL), the atmospheric layer closest to the ground. The winds above the PBL travel faster and farther because they are not slowed by friction with the ground.
Analysis of long-range airflows on the days the samples were collected, combined with chemical analysis, revealed that the dust particles had traveled a distance of 1,200 miles (1,900 kilometers) and originated in China.
Chemical analyses of the samples revealed traces typical of agricultural areas, such as animal manure, pesticides and fertilizers, as well as rare elements such as zirconium and hafnium, which are mined in that part of China.
The microbes were embedded in the particles, which protected them from ultraviolet light and desiccation, allowing some to remain viable. The human pathogenic species included bacteria such as I coli, Staphylococcus saffron And Clostridium difficile.
Rodó said the research team was surprised to find so many different microbes, since the original goal of the study was to analyze the chemistry of dust particles, which can reach the ground by falling or in raindrops.
“The identification of pathogenic organisms above the PBL indicates that large parts of the troposphere may become potential reservoirs and act as long-distance spreaders of a rich diversity of microbes,” the researchers concluded.
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Rodó said: “We are talking about ultra-low concentrations, and in most cases they would not cause infection. But we cannot rule that out in immunocompromised individuals.”
It has been shown that viable bacteria and fungi can travel long distances in soil dust, for example from Africa to the CaribbeanBut the researchers said: “The isolation of species harmful to humans had never been reported over such large distances (until now).”
Dr Allen Haddrell, from the University of Bristol, UK, who was not part of the research team, said: “Many studies have reported antimicrobial resistance genes in the air. The (new study) shows that there is a physical way in which AMR genes can spread extremely long distances.
“In addition, the genes are transported in living organisms, so they are more likely to pass through once the aerosol settles. In the long term, this will be a problem.” Many experts have warned that antibiotic resistance poses a serious threat to humanity.
Prof Chris Thomas from the University of Birmingham, UK, said: “The chance of getting an infectious dose (via strong winds) must be considerably less than meeting an infected person on a plane, or even just on holiday to a foreign country. The study also implies that the dust and chemicals carried through the air may be more harmful (as air pollution) than the microbes.”