The microbial menagerie in the air


Novel study reveals the huge variety of organic materials people encounter every day – and the ubiquity of insect repellent. Andrew Masterson reports.


Some examples of the airborne particles captured by filters worn by volunteers.

Some examples of the airborne particles captured by filters worn by volunteers.

Jiang, et al

A novel experiment tracking the microbes to which humans are exposed as they live their lives has identified at least 2560 species, falling into 44 phyla.

The research, conducted by a team led by Chao Jiang, from the Stanford University School of Medicine in the US, represents the most detailed examination ever of the vast array of organic material people encounter in the air – collectively known as the “exposome”.

In a paper published in the journal Cell, the scientists conclude that human exposomes are diverse, continually changing networks with the potential impact human health.

To make their findings, the researchers attached specially constructed filters to 15 volunteers, and monitored them for 890 days across 66 geographical locations. The filters were capable of trapping samples as small as 10 bacterial cells, and were collected at least once a week.

Using extracted DNA and RNA, compared against a reference bank of 40,000 species, Jiang and colleagues identified an extraordinary range of exposome sources, including bacteria, viruses, pollen, fungal spores, hair, and skin flakes.

The research found that 78% of the species detected belonged to two fungal, four bacterial and three plant or animal phyla. Perhaps not surprisingly, DNA from domestic pets was very common, as were traces from different types of mite and common insects such as bees, flies and mosquitoes.

Perhaps less expected, RNA from viruses associated with such insects were also detected. Present in surprisingly large quantities were rotifers – a group of microscopic asexual animals that lives in water and soil.

The filters used by the volunteers also detected non-living, or abiotic, exposome sources. From 2796 identified chemical formulae, the researchers identified 972 compounds – the overwhelming majority of them matching samples in a toxicant database but absent from one listing naturally occurring substances.

Three, in particular, were present in every sample: the insect repellent diethyltoluamide (DEET), the pesticide omethoate, and the carcinogen diethylene glycol (DEG).

DEET is not recommended for use where it can be breathed in, but Jiang and colleagues say their filter finds indicate that its presence means that it could feasibly be inhaled and reach the lower respiratory tract.

“Our findings thus revealed a previously unrecognised type of potentially hazardous exposure that is commonly detected in the air,” they write.

Over all, the scientists conclude that their results indicate “a huge gap exists between the complexity of our environmental exposures and what is presently in our knowledge database”.

Further research could help understand not only the mechanics of seasonal allergies, such as hay fever, but also the environmental risk exposures for many diseases, including types of cancer.

  1. https://doi.org/10.1016/j.cell.2018.08.060
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