Antibiotic resistance genes float in the clouds

The fact that bacteria adapt to resist antibiotics is a natural evolutionary phenomenon. But the massive use of these drugs to treat human, animal and plant infections has greatly amplified it. And every year, a good proportion of the tons of antibiotics used end up in the environment, namely in soil, rivers, oceans, etc. Resistant bacteria can develop there, transmit their resistance genes to other bacteria and disperse with the wind and reach high altitudes. Although the atmosphere is not a suitable environment for their survival, fragments and genetic material can still reach the clouds, travel from one continent to another and return to dry land through rain.

Clouds vacuumed up into test tubes

At the Puy-de-Dôme weather station located at an altitude of 1,465 meters in the French Massif Central, Franco-Canadian researchers spent two years taking 12 "cloud samples" with a special vacuum cleaner aimed at nimbus and cumulus clouds. For each sample, they measured the quantity of bacteria and 33 resistance genes corresponding to the main antibiotics used today. Of these, 29 were detected at least once and 6 were observed in at least 75% of the samples. The clouds contained an average of 8,000 bacteria mainly of plant origin, of which 5 to 50% could be alive and potentially active, and over 20,000 copies of antibiotic resistance genes per milliliter of water.^1,2

The researchers found that the distribution of these genes varied according to the geographical origin of the air masses sampled. For example, genes for resistance to quinolones (antibiotics whose use has been restricted for several years due to the antibiotic resistance they promote) were more abundant in high ocean clouds. Sulfonamide and tetracycline resistance genes were more prevalent in clouds formed over continental surfaces, perhaps due to their widespread use in livestock farming.

Reservoirs of antibiotic resistance genes hovering over our heads

The researchers extrapolated their measurements to the total volume of clouds around the earth, assuming that all would have the same concentration of antibiotic resistance genes. The results showed that every year, around 70 trillions of trillions (1024) of these genes pass through the clouds, of which around 3% could potentially fall back down to the earth's surface.

This study highlights the role of the atmosphere as one of the routes by which antibiotic resistance factors are disseminated worldwide. Additional studies to pinpoint the sources of bacterial emissions could help limit their dispersion.