Antibiotic-resistant bacteria have been found in clouds

Antibiotic resistance in bacteria is becoming a major public health concern worldwide. The extensive use of antibiotics to sustain human activities has led to the acquisition of antibiotic-resistance genes in bacteria and their massive spread into the environment. 

Now, by sampling clouds at an atmospheric research station on top of the Puy de Dôme summit, a dormant volcano in France’s Massif Central, a team of researchers led by the Laval University in Canada and the University of Clermont Auvergne in France has found that the atmosphere can be a large-scale dissemination route for bacteria carrying antibiotic-resistance genes.

“This is the first study to show that clouds harbor antibiotic resistance genes of bacterial origin in concentrations comparable to other natural environments,” said lead author Florent Rossi, a postdoctoral fellow specializing in bioaerosols at Laval.

The scientists conducted 12 cloud sampling sessions over a period of two years using high-flow rate “vacuums.” The analysis of these samples revealed that, on average, they contained approximately 8,000 bacteria per milliliter of cloud water. Moreover, between five and 50 percent of them could be alive and potentially active. 

“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,” Rossi explained.

By using this data, the experts measured the concentration of 29 sub-types of antibiotic-resistance genes found in atmospheric air masses, and discovered that clouds contained an average of 20,800 copies of antibiotic-resistance genes per millimeter 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,” Rossi said.

Although airborne transport of such genes is a natural phenomenon, the massive use of antibiotics in agriculture and medicine has played a major role in the proliferation of these resistant strains and their widespread dissemination in the environment. 

“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,” Rossi concluded.

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

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By Andrei Ionescu, Earth.com Staff Writer

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