Researchers Discover that the Pleasant Odor of Wet Soil Signals Danger to Bacteria-Eating Worms

Geosmin has a distinct odor that pervades the air after a summer rainstorm or fills your nose when gardening. It’s the earthy, almost comfortable scent of moist soil.

However, as a recent study in the journal Applied and Environmental Microbiology reminds out, that odor has a specific purpose. It is produced by bacteria that are well-known toxin makers.

This serves as a warning to C. elegans, a common type of worm, that the bacteria they’re going to eat is toxic. The chemical is an aposematic signal that activates the blind worm’s sense of taste, similar to how the vivid colors of a caterpillar or the spines of a pufferfish warn a sighted predator to keep away.

Nematodes like C. elegans are only a few centimetres long yet can be found all across the world, including Antarctica. Nematodes are also the most common animal on the planet, accounting for around four-fifths of all animals.

Because their biological processes are less complex but similar to those of humans, researchers frequently employ C. elegans as a model organism in their research.

“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 has no effect on the cells’ ability to grow, eat, or divide. It doesn’t directly deter predators. It appears to be merely a cautionary note. He claims he isn’t aware of any other bacteria-produced compounds that behave similarly.

It was definitely not obvious. 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.

Liana Zaroubi

Liana Zaroubi (MSc 21), a former Findlay student who is now doing her Ph.D. at Simon Fraser University, led the research.

A scent of danger

Zaroubi admits that she required some time to get to the conclusion 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.”

To test their idea, the researchers employed several strains of C. elegans. They first studied the movement and behavior of worms on agar plates containing geosmin but no bacteria. In this case, the worms reacted negatively to the compound’s presence by moving quickly and changing directions frequently.

Mutant nematodes lacking the chemosensory ASE neuron, which is responsible for taste, exhibited normal behavior. C. elegans appeared to be unaffected by geosmin.

Streptomyces coelicolor microorganisms were added in a separate experiment. When the worms detected the presence of geosmin in their prey, the researchers saw them avoiding it.

Those without the ASE neurons, on the other hand, devoured the deadly germs, killing both predator and prey.

A taste of evolution

Geosmin is an extremely smelly chemical that may be detected by humans at a concentration of five parts per trillion. While many people enjoy its odor, it is a bacteria-created contaminant in human drinking water that can make water taste like dirt.

The full scope of geosmin’s biological use is still unknown. The researchers believe, however, that the molecule can help them understand how bacteria and their predators interact, as well as how sophisticated behaviors like toxin avoidance emerge.

Along with Zaroubi and Findlay, Ph.D. student Imge Ozugergin, Karina Mastronardi (PhD 21), Ph.D. candidate Anic Imfeld, Chris Law at the Centre for Microscopy and Cellular Imaging, professor Yves Gélinas from the Department of Chemistry and Biochemistry and associate professor Alisa Piekny from the Department of Biology contributed to the study.