World's First 'Atlas' of Airborne Microbes Shows Pathogens in Urban Areas

Locations where air samples and environmental data were collected around the globe. Credit: Hong Kong Polytechnic University

Most people outside the scientific profession were not thinking about the 10,000+ bacteria that litter every cubic meter of air. That is, until everyone became overly aware that some of those bacteria could be SARS-CoV-2.

"Having experienced the pandemic for three years, people now pay more attention to this invisible but influential microbial community,” said Xiang-dong Li, professor at Hong Kong Polytechnic University and author of a new study that has compiled the most comprehensive map of the world's airborne microbes to date.

While the total number of microbes in the sea or soil is thousands of times larger than those in the air, the research showed the diversity of aerial microbes is just as high. Additionally, half of the airborne bacteria originate from surrounding environments and are mainly influenced by local meteorological and air quality conditions—including human activities.

In collaboration with Chinese researchers, as well as scientists at Michigan State University, the Hong Kong Polytechnic University-led team spent a year sampling airborne microbes around the globe.

Altogether, the team analyzed the bacterial communities of 370 individual air particulate samples collected from 63 global sites—ranging from those at ground level (1.5 to 2 m high) to rooftops (5 to 25 m high) and high mountains (5,238+ m), as well as from densely populated urban centers to the Arctic Circle.

According to the study published in PNAS, genetic analysis showed that the core microbial communities in the air were not the same as those in marine or soil ecosystems. In fact, even though air is not a defined structure with boundaries, like soil, the researchers found that the core bacterial communities were still distinctly localized and stable.

While the team did not find any significant disparities in richness between urban and non-urban areas, the abundance of potential pathogens was significantly higher in urban air. This was particularly the case for pathogens with the highest risk of mortality, including Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species.

In fact, the researchers said 22% of identified airborne pathogens (or 37 species) only occurred in urban areas. Additionally, the alteration of the airborne bacterial taxonomic composition due to urbanization led to a change in some phenotypic characters.

“We have verified that human activities have certainly changed the structure of microbiomes in the natural ambient air. The inhalable infectious bacteria that proliferate in cities are particularly of concern considering rapid urbanization and our growing understanding of airborne contagion, spurred by COVID-19 research,” said Li.

Surprisingly, the scientists found that half of airborne bacteria originate from ground sources. Overturning previous assumptions, they showed vegetation is not the main terrestrial source of airborne bacteria. Instead, the crashing of waves, the shaking of leaves, and even frequent activities and constant respiration of animals and humans were found to be bigger drivers of bacterial exchange between the surface and the air.

Location-wise, moving from the equator, microbial diversity maximizes at mid-latitudes before falling away again. This pattern was already well established for terrestrial and aquatic microorganisms, but the new atlas confirms it applies to the airborne microbiome, as well. The researchers believe the diversity increase seen at mid-latitudes is caused by stronger sources of microbial input to those regions.

Li says the study results can be used as a future reference for predicting microbiome responses and the human health impacts of inhalable microbiomes.

“The close relationship between modern human activities and the microbes around us underscores the need to predict future changes accurately,” he said.