The pleasant smell of wet soil indicates danger to bacteria-eating
worms, researchers find
The chemical compound geosmin's powerful taste warns predators to keep
away from certain microbes
Date:
April 5, 2022
Source:
Concordia University
Summary:
Researchers have found that geosmin is made by certain kinds of
bacteria that are known toxin producers. This acts as a warning to
C. elegans, a common type of worm, that the bacteria they are about
to graze on is poisonous. The chemical is an aposematic signal that
triggers the blind worm's sense of taste just like a caterpillar's
bright colors or a pufferfish's spines tell a sighted predator to
stay away.
FULL STORY ==========================================================================
The smell of geosmin is unmistakable: it's the odour that permeates the
air after a summer rain squall or fills your nose while gardening. It's
the smell of wet soil -- an earthy, almost comforting scent.
==========================================================================
But as a new study just published in the journal Applied and Environmental Microbiology points out, that smell also has a particular purpose. It is
made by certain kinds of bacteria that are known toxin producers. This
acts as a warning to C. elegans, a common type of worm, that the
bacteria they are about to graze on is poisonous. The chemical is an
aposematic signal that triggers the blind worm's sense of taste just
like a caterpillar's bright colours or a pufferfish's spines tell a
sighted predator to stay away.
Just a few millimetres long, nematodes like C. elegans are tiny but
found all over the Earth, including Antarctica. Nematodes are also the
most abundant animal on Earth, accounting for about four-fifths of the
global animal population. Researchers often use C. elegans as a model
organism in their studies because their biological systems are less
complex but similar to those of humans.
"Through our study, we found that geosmin in Streptomyces coelicolor,
a bacteria that is toxic to C. elegans, does not appear to have any role
other than as a signal," says Brandon Findlay, associate professor in
the Department of Chemistry and Biochemistry and the paper's supervising author. "It doesn't help the cells grow, eat or divide. It doesn't ward
off predators directly. It just seems to be there as a warning." He
says he is unaware of any other bacteria-produced chemicals that act in
that fashion.
The study was led by Liana Zaroubi (MSc 21), one of Findlay's students
who is now pursuing her PhD at Simon Fraser University.
A scent of danger Zaroubi acknowledges that it took her some time to
arrive at the idea that geosmin was aposematic.
==========================================================================
"It was definitely not obvious," she says. "I eliminated many hypotheses
before finding that geosmin acted as a warning signal. However, each
ruled-out experiment revealed important clues that helped elucidate the
mystery that is geosmin. We followed the science and I believe that
was key to this discovery." The researchers used several strains of
C. elegansto test their hypothesis.
First, they observed the movement and behaviour of worms on agar plates
where geosmin was present but bacteria was absent. In this instance, the
worms reacted adversely to the presence of the compound, moving rapidly
with frequent changes in direction. However, mutant nematodes without
the chemosensory ASE neuron, which is dedicated to taste, were observed
to behave normally. Geosmin itself appeared to be non-toxic to C. elegans.
A separate experiment was designed that included Streptomyces coelicolorbacteria. The researchers observed the worms avoiding their
prey when they could taste the presence of geosmin. But those without
the ASE neurons devoured the toxic bacteria, with predictably fatal consequences for both predator and prey.
A taste of evolution Geosmin is a very pungent compound, detectable by
humans at five parts per trillion. While many find its odour pleasant,
it is also a common bacteria- created contaminant in human drinking
water that can cause water to taste like dirt.
The extent of geosmin's biological uses is still not completely
understood.
However, the researchers believe the compound offers insight into how
bacteria and their predators interact and how complex behaviours like
toxin avoidance evolve.
Along with Zaroubi and Findlay, PhD student Imge Ozugergin, Karina
Mastronardi (PhD 21), PhD candidate Anic Imfeld, Chris Law at the Centre
for Microscopy and Cellular Imaging, professor Yves Ge'linas from the Department of Chemistry and Biochemistry and associate professor Alisa
Piekny from the Department of Biology contributed to the study.
========================================================================== Story Source: Materials provided by Concordia_University. Original written
by Patrick Lejtenyi. Note: Content may be edited for style and length.
========================================================================== Journal Reference:
1. Liana Zaroubi, Imge Ozugergin, Karina Mastronardi, Anic Imfeld,
Chris
Law, Yves Ge'linas, Alisa Piekny, Brandon L. Findlay. The Ubiquitous
Soil Terpene Geosmin Acts as a Warning Chemical. Applied and
Environmental Microbiology, 2022; DOI: 10.1128/aem.00093-22 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/04/220405143530.htm
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