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Bacteria use raindrops to get a free ride

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Raindrops can help transport pathogenic bacteria from the soil to the air, which helps explain how bacteria can travel long distances, according to a new study out of the US. Bacteria get caught up in bubbles on the surface of raindrops. When the bubbles burst, the bacteria are launched up in the air in tiny aerosols which are light enough to be transported by wind. The authors found that the bacteria could stay alive for more than an hour in these aerosols, which could have serious implications for agriculture and health as the bacteria can travel much faster compared to any other transport method.

Journal/conference: Nature Communications

Organisation/s: Massachusetts Institute of Technology, USA

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From: Springer Nature

Rainfall can mobilise microbes

Bacteria in soil may be dispersed in the air via raindrops according to a study published in Nature Communications this week. This is a new mechanism that sheds light on our understanding of how bacteria may be spread over long distances.

Previous work has shown that as a raindrop impacts soil, aerosols (suspended water droplets) are generated. Soils can act as an intermediate home for bacteria, but it has been unclear how bacteria might be transferred to the atmosphere as it was believed that they would not survive the aerosolization process.

Using high-speed cameras, fluorescent imaging and modelling experiments, Cullen Buie and colleagues find that a single raindrop can transfer 0.01% of bacteria on the soil surface to the atmosphere, where it can survive for more than one hour. Although the percentage of bacteria transferred to the atmosphere seems low, the team calculates that global precipitation may transport between 1.6% and 25% of the total bacteria from land depending on differing soil types and local climate. Aerosolization is visualized in three non-pathogenic strains of soil bacteria.

Although these findings explain how bacteria can be transferred to the atmosphere, which has implications for the climate, agricultural productivity and human health, there is no evidence that this mechanism promotes diseases after heavy rain.

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    Movie 1: Aerosols generated by drop impingement on a porous surface, which maximized the aerosol generation. The porous surface served as an ideal soil-like surface. The white lines are the trajectories of aerosols ejected from the initial droplet after impact. Due to air flow above the droplet, the trajectories of the ejected aerosols are curved. The scale bar indicates 1 mm. Credit: Joung et al. Nature Communications
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    Movie 2: Bubble formation at the interface of surface and raindrop. The number of bubbles formed inside a raindrop is the key factor influencing bacterial transfer. With increasing bubbles formed inside a raindrop, more bacteria can be transferred by bubble bursting. Credit: Joung et al. Nature Communications

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