Microbes that cause disease in humans can travel thousands of kilometers on high-level winds, scientists have revealed for the first time.
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 were shown to be alive – in other words, they survived the long journey and were able to replicate.
The researchers said it was unlikely that this intercontinental transport route would directly cause disease in humans, because the concentration of microbes was low. However, they said it was a cause for concern that microbes could be seeded into new environments and that antibiotic resistance genes could travel this way.
The study showed the microbes traveled 1,200 miles (2,000 km) on dust particles blown from farmland in the Northeast China to Japan. Similar patterns of winds exist around the world. More than 300 types of bacteria and about 260 types of fungi were found in the samples collected across Tokyo. Other microbes not yet known to science are thought to be present.
Prof Xavier Rodó at the Barcelona Institute for Global Healthwho led the research team, said: “About 30-40% of the microbes were potentially pathogenic species, either well-recognized human pathogens or opportunistic pathogens [which affect people with weakened immune systems].”
The study “is a word of warning that we need to change our view of the air”, Rodó added, especially the idea that air at higher altitudes is almost sterile.
“We should use the new methods to take samples and see what’s 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 samples of dust 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 move faster and further, as they are not slowed down by friction with the ground.
Analysis of the long-range air currents on the days the samples were collected, combined with chemical analysis, showed the dust particles traveled 1,243 miles and came from China.
The chemical analyzes of the samples showed characteristic features of agricultural areas, including animal manure, pesticides and fertilizers, and also rare elements such as zirconium and hafnium, which are mined in this part of China.
The microbes were embedded in the particles, which protected them from ultraviolet light and dehydration, allowing some to remain viable. The human pathogen species has bacteria such as E coli, Staphylococcus saprophyticus and Clostridium difficile.
Rodó said the study team was surprised to find so many different microbes, since the initial goal of the research was to analyze the chemistry of the dust particles. Such particles can reach the ground by falling or in raindrops.
“The identification of pathogenic organisms above the PBL suggests that large portions of the troposphere may become potential reservoirs and may act as long-distance dispersers of a rich variety of microbes,” the researchers concluded.
to newsletter promotion
Rodó said: “We are talking about ultra-low concentrations, and in most cases it will not cause infection. But we cannot rule it out in immunocompromised individuals.”
Viable bacteria and fungi have previously been shown to travel long distances in raw material from Africa to the Caribbean. But the researchers said: “The isolation of species harmful to humans has never before been reported over such long distances [until now].”
Dr Allen Haddrell, at 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 means by which AMR genes can spread extremely long distances.
“Furthermore, the genes are carried in living organisms, so they have a greater probability of succeeding once the aerosol has subsided. In the long term, this is going to be a problem.” Many experts have warned against this Antibiotic resistance is a serious threat to humanity.
Prof Chris Thomas, at the University of Birmingham, UK, said: “The chances of getting an infectious dose [via high-level winds] should be significantly less than if you encounter an infected person on a plane, or even just go on vacation to a foreign country. The study also implies that the dust and chemicals carried in the air may be more harmful [as air pollution] than the microbes.”
