Massive amounts of antibiotic resistance genes found in clouds

Each year, tens of thousands of tons of antibiotics are used for healthcare purposes in humans, animals, and plants, a portion of which is released into the environment. Antibiotics are thus found not only in wastewater, but also in rivers, the sea, and soil, enabling environmental bacteria to acquire resistance genes that can then be transferred to other bacteria. Bacterial cells and genetic material can aerosolize and, due to air turbulence, rise into the atmosphere, travel long distances, and then become part of the water cycle.

Proof of a “resistome” in the clouds

A French and Canadian research team assessed the quantity of antibiotic resistance genes in clouds at the Puy de Dôme meteorological observatory in France’s Massif Central, at an altitude of 1,465 meters. Twelve samples were taken with a high-flow vacuum between September 2019 and October 2021, revealing an average concentration of around 5,400 copies of resistance genes per cubic meter of air (measured by flow cytometry). The 33 resistance genes found correspond to the main antibiotic families in use today: quinolones, sulfonamides, tetracyclines, aminoglycosides, glycopeptides, β-lactam antibiotics, and macrolides. Of these, 29 were detected at least once, and 6 were observed in at least 75% of samples. 

Rather than varying with the seasons, the distribution of these genes varied according to the geographical origin of the air masses. Resistance genes for the quinolone family, antibiotics strongly implicated in antibiotic resistance and whose use has been restricted in Europe since 2018, were more abundant in high marine clouds. Resistance genes for the sulfonamide and tetracycline families were more abundant in clouds formed over continental surfaces, perhaps due to their widespread use in livestock farming.

The atmosphere is a major highway for resistome dissemination

Considering the average concentration of resistance genes in clouds according to the study (5,400/m3 of air), the researchers estimate that clouds permanently carry around 2.53 × 1021 copies of resistance genes worldwide. Thus, each year, between 1.29 × 1025 and 2.06 × 1026 resistance genes may be transported by clouds, with a very large quantity of these genes (2.2 ×1024) falling to earth through precipitation (and a fraction remaining evaporated in the atmosphere). 

This study has found the atmosphere to be one of the routes by which antibiotic resistance factors are disseminated worldwide. Studies that pinpoint the sources of bacterial emissions may limit their dispersal^.2 

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