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  • From: Lawrence London <lfljvenaura@gmail.com>
  • To: permaculture <permaculture@lists.ibiblio.org>
  • Subject: [permaculture] Watch Raindrops Catapult Bacteria Into The Air : Shots - Health News : NPR
  • Date: Wed, 8 Mar 2017 16:21:32 -0500

http://www.npr.org/sections/health-shots/2017/03/07/518811502/watch-raindrops-catapult-bacteria-into-the-air-and-its-beautiful

WATCH: Raindrops Catapult Bacteria Into The Air, And It's Beautiful

March 7, 201712:14 PM ET

Rae Ellen Bichell <http://www.npr.org/people/488507788/rae-bichell>

When you step outside after a big rainstorm and take a deep whiff of that
fresh, earthy smell, you're mostly smelling a chemical called geosmin.

It's a byproduct of bacteria and fungi. And something about rain lofts
<https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3013058/> the chemical — and
sometimes the organisms themselves — into the air, a process that not only
helps release that earthy smell but may, in very rare conditions, spread
diseases.

Somehow raindrops launch tiny living things off the ground.

Wind can pick bacteria up from the ground and bring into the sky,
sometimes settling
in clouds
<http://www.npr.org/2013/01/29/170459317/bird-plane-bacteria-microbes-thrive-in-storm-clouds>
thousands of feet above us. Sea spray can also send
<http://journal.frontiersin.org/article/10.3389/fmicb.2014.00557/full>
bacteria flying. The mystery has been, how does rain do it?

Now, mechanical engineers writing
<http://nature.com/articles/doi:10.1038/NCOMMS14668> Wednesday in the
journal *Nature Communications *say they've figured it out. It has to do
with tiny bubbles.

Using high-speed cameras and fluorescent dye, the researchers filmed drops
of water as they fell on different types of soil infused with bacteria.
They watched as the drops delicately catapulted the microbes into the air.
Aerosols

The white lines are the trajectories of tiny water droplets ejected when a
raindrop hits the ground. Airflow in the room pushes the droplets off to
the side.
[image: Aerosols]

Source: Joung et al. Nature Communications

When a raindrop hits the ground at the right speed, it traps air bubbles
beneath it, each one no wider than a human hair.

Like an air bubble stuck under a container at the bottom of a pool, those
bubbles then rise, bursting when they reach the surface.

As the bubbles burst, they send a spray of tiny water jets into the air,
sometimes carrying bacteria with them.

"It's very similar to what happens if you pour a glass of champagne or any
carbonated beverage and you see that fizz on top," says Cullen Buie
<http://web.mit.edu/lemi/pers_faculty.html>, a mechanical engineer at the
Massachusetts Institute of Technology and an author on the paper. In that
case, the popping bubbles are full of carbon dioxide; in this case, they're
full of air.

In a few microseconds, Buie and his colleagues found, a single raindrop can
create hundreds of tiny airborne droplets, each one carrying as many as
several thousand live bacteria.

It's as if the ground has gently sneezed.
Bubbles

The more bubbles there are, the more chances there are for bacteria to get
launched into the air.
[image: Droplet]

Source: Joung et al. Nature Communications

The soaring microbes can survive inside a tiny water droplet for at least
an hour.

"The next step is studying how far they can potentially travel," says
Cullen, who estimates that bacteria could travel for miles, if the winds
are right and they don't get dragged down by other raindrops.

The three kinds of soil bacteria involved in this study — *Corynebacterium
glutamicum, Pseudomonas syringae, Bacillus subtilis* — are harmless. But
pathogens can also get a lift <http://aem.asm.org/content/35/2/290.short>
from water droplets.

Cullen and his colleagues got started on this study after a British
scientist contacted them after noticing an uptick
<http://www.nature.com/nrmicro/journal/v4/n4/abs/nrmicro1385.html> in a
rare disease called melioidosis during the rainy season in parts of
Southeast Asia and northern Australia. The lung infection is caused by the
soil-dwelling bacterium *Burkholderia pseudomallei*.

Cullen's results suggest rain could be a reason for that uptick. He and his
colleagues varied the speed of the falling raindrops and the type and
temperature of the soil. They found that the Goldilocks conditions for
producing spray loaded with bacteria is exactly what you'd find in certain
tropical places — sandy clay soils, a soil temperature of 86 degrees
Fahrenheit and droplets falling at about 3 miles an hour.

"I don't want people thinking they shouldn't go outside after it rains
because they might get some bacterial infection. I definitely don't want
people thinking that," says Cullen.

Melioidosis, he says, is very rare. And, if rain really is the culprit
behind a small rise in cases, that would only be the case in the rare
locations with exactly the right conditions — and with the disease-causing
bacterium living in the soil.

"Bacteria, unfortunately, get a bad rap," says Buie. Most, he says, are
harmless or even beneficial. And sometimes they even smell good.



  • [permaculture] Watch Raindrops Catapult Bacteria Into The Air : Shots - Health News : NPR, Lawrence London, 03/08/2017

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