Microorganisms can travel long distances in the troposphere

Pathogens can be airborne, but little is known about the diversity of microbes that can survive at very high altitudes, where the conditions are harsh. "We know that above a certain point in the troposphere (called the planetary boundary layer), certain materials can be transported over long distances because the air in that region is isolated from the surface and there is less friction. But we did not suspect that viable microorganisms could be there also," says Xavier Rodó, ICREA researcher at ISGlobal.

"Our study is unique in that we performed 10 tropospheric flights to examine microbial diversity at high altitudes, while most studies have been performed only a few metres above the ground or the ocean," he adds.

Airborne microbes high above Japan

Using a Cessna aircraft, Rodó and an international team of colleagues conducted ten air surveys between 1,000 and 3,000 metres above Japan, starting from Chofu airport near Tokyo. All flights were planned to follow wind currents coming from mainland Asia in what are known as tropospheric bridges, which connect air from distant regions of the world; in this case, air that uplifts in mainland China and then descends over Tokyo due to typical winter weather conditions. For comparison, samples were also collected on the ground at Chofu. A total of 22 aerosol filter samples, collected during two periods (February and April, 2014), were analysed for their chemical and biological composition.

DNA sequencing allowed the research team to identify over 266 fungal and 305 bacterial genera associated with the aerosols, some of which are potentially pathogenic for humans, other animals or plants. For instance, bacterial species such as Escherichia coli, Serratia marcescens, Clostridium difficile, Clostridium botulinum, Haemophillus parainfluenzae, Acinetobacter baumannii and several Staphylococcus species were identified, as well as fungal species from genera such as Candida, Cladosporium and Malassezia, capable of causing disease in susceptible and immunocompromised individuals.

By culturing some of the samples, the researchers showed that bacteria collected from the air remained viable, and that some were resistant to commonly used antibiotics. "Surprisingly, the Micrococcus luteus strain isolated was resistant to multiple drugs, including carbapenems, glycopeptides, ciprofloxacin, and trimethoprim-sulfamethoxazole. Our findings suggest that antimicrobial resistance could spread over long distances via this previously unrecognised route," says Sofya Podzniakova, co-first author of the study.

Travelling thousands of kilometres

The association of these aerosols with certain elements such as zinc sulfate and potassium, commonly used in fertilizers and pesticides, suggests an agricultural origin, consistent with intensively farmed croplands in northeast China.

Finally, during the days sampled, flight and ground samples were very similar in terms of microbial diversity, which can be explained by air descending from high altitudes to the ground. The particle transport models, simulated by Roger Curcoll -currently a researcher at the Universitat Politècnica de Catalunya -- BarcelonaTech (UPC)- have corroborated both the possible transport of these particles from northeast China and the decline of tropospheric air masses to land in Japan.

"Our findings uncover a rich and unprecedented diversity of microbes that are dispersed by wind currents thousands of kilometres away from their sources by intense tunnels of wind that form high in the troposphere," says Rodó. "They represent a paradigm shift in our understanding of how human health can be affected by pathogens thriving in the environment, particularly in the air."

While the study does not prove a causal link between the presence of known human pathogens in aerosols and health effects, it does emphasise the need to further explore the spread of different microbial pathogens over long distances.