Researchers sampled fluffy clouds and found 20,800 copies of antibiotic-resistance genes

The genes were hiding in plain sight

The research team sampled clouds at the Puy de Dôme summit, a dormant volcano in France's Massif Central. They conducted 12 cloud sampling sessions over two years using high-flow rate "vacuums" from an atmospheric research station 1,465 meters above ground.

The analysis revealed that the cloud samples were hosts to 8,000 bacteria per milliliter of cloud water, on average.

"These bacteria usually live on the surface of vegetation or soil. They are aerosolized by the wind or by human activities, and some of them rise into the atmosphere and participate in the formation of clouds," explained Rossi. 

The scientists were able to measure the concentration of 29 subtypes of antibiotic-resistance genes carried in atmospheric air masses. On average, the clouds contained 20,800 copies of antibiotic-resistance genes per milliliter of cloud water. 

"Oceanic clouds and continental clouds each have their signature of antibiotic resistance genes. For example, continental clouds contain more antibiotic resistance genes used in animal production," said Rossi. 

The concentrations are, however, variable. They range from 330 to more than 30,000 bacteria per milliliter of cloud water. And between five percent and 50 percent of these bacteria could be alive and potentially active. 

"Our study shows that clouds are an important pathway for antibiotic-resistance genes spreading over short and long ranges. Ideally, we would like to locate emission sources resulting from human activities to limit the dispersal of these genes," added Rossi.

The study was published in the journal Science of The Total Environment. 

Study Abstract:

Antibiotic resistance in bacteria is becoming a major sanitary concern worldwide. The extensive use of large quantities of antibiotics to sustain human activity has led to the rapid acquisition and maintenance of antibiotic resistant genes (ARGs) in bacteria and to their spread into the environment. Eventually, these can be disseminated over long distances by atmospheric transport. Here, we assessed the presence of ARGs in clouds as an indicator of long-distance travel potential of antibiotic resistance in the atmosphere. We hypothesized that a variety of ARGs can reach the altitude of clouds mainly located within the free troposphere. Once incorporated in the atmosphere, they are efficiently transported and their respective concentrations should differ depending on the sources and the geographical origin of the air masses. We deployed high-flow rate impingers and collected twelve clouds between September 2019 and October 2021 at the meteorological station of the puy de Dôme summit (1465 m a.s.l., France). Total airborne bacteria concentration was assessed by flow cytometry, and ARGs subtypes of the main families of antibiotic resistance (quinolone, sulfonamide, tetracycline; glycopeptide, aminoglycoside, β-lactamase, macrolide) including one mobile genetic element (transposase) were quantified by qPCR. Our results indicate the presence of 29 different ARGs' subtypes at concentrations ranging from 1.01 × 103 to 1.61 × 104 copies m−3 of air. Clear distinctions could be observed between clouds in air masses transported over marine areas (Atlantic Ocean) and clouds influenced by continental surfaces. Specifically, quinolones (mostly qepA) resistance genes were prevalent in marine clouds (54 % of the total ARGs on average), whereas higher contributions of sulfonamide, tetracycline; glycopeptide, β-lactamase and macrolide were found in continental clouds. This study constitutes the first evidence for the presence of microbial ARGs in clouds at concentrations comparable to other natural environments. This highlights the atmosphere as routes for the dissemination of ARGs at large scale.