Smell of wet soil signals danger to bacteria-eating worms

A new study led by Concordia University in Canada has found that geosmin – a pungent chemical compound present in wet soil – acts as a warning (or “aposematic”)  signal to certain species of worms such as Caenorhabditis elegans. The signal from the wet soil indicates that the bacteria the worms are about to feed on are poisonous and should be avoided. 

Thus, the presence of geosmin triggers blind worms’ sense of taste just like a caterpillar’s bright colors or a pufferfish’s spines signal the presence of predators. These warning signals tell the animals to keep their distance if they want to survive.

The scientists used several types of C.elegans – a few millimeters-long nematode found all over the world, including the poles – to test this hypothesis. Initially, they observed the movements and behaviors of worms on agar plates containing geosmin, but no traces of bacteria. The worms reacted adversely to the chemical compound, moving rapidly with frequent changes in direction. Interestingly, mutant nematodes that had their taste-related neurons removed behaved normally in the presence of geosmin. 

In a second experiment, the researchers included strains of Streptomyces coelicolor bacteria on the plates (a type of bacteria that is dangerous and often fatal to C.elegans), and discovered that worms that were concomitantly exposed to geosmin (which in itself is not toxic to them) avoided feeding on the bacteria. However, worms with their taste neurons removed eagerly devoured the toxic bacteria, with predictably fatal consequences.

“Through our study, we found that geosmin in Streptomyces coelicolor, a bacterium that is toxic to C. elegans, does not appear to have any role other than as a signal,” explained study senior author Brandon Findlay, an associate professor of Chemistry and Biochemistry at Concordia. “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.”

According to Professor Findlay and his colleagues, although the extent of geosmin’s biological uses is not yet completely understood, this study offers fresh insight into how bacteria and their predators interact and how complex behaviors such as toxin avoidance evolve.

“Geosmin thus appears to act as a warning signal indicating the unpalatability of its producers and reducing predation in a manner that benefits predator and prey. This suggests that molecular signaling may affect microbial predator-prey interactions in a manner similar to that of the well-studied visual markers of poisonous animal prey,” the authors concluded.

The study is published in the journal Applied and Environmental Microbiology.

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By Andrei Ionescu, Earth.com Staff Writer