▎ WuXi AppTec content team editor
After a heavy rain, or shortly after the meadows are poured, there is a special smell in the air, which is very fresh and even pleasant to the human senses. This feeling is mainly related to a class of molecules called soil stinkins, which are produced by bacteria in the soil.
Image credit: 123RF
For humans, after rain and sunshine, they can go out to forage for food or work, and this "earthy taste" has also become a signal of relaxation. However, in places where we can't see them, countless nematodes don't like these smells. Instead, these signals portend the presence of danger to nematodes. They don't approach nature to smell the aroma of this earth odorin, but quickly move away from the source of these smells.
Nematodes, which are only a few millimeters long, are almost all over the world, and they can be seen even in Antarctica. They usually feed on bacteria in large numbers, but nematodes do not have a complex physiological system, and it is not like humans who have eyes to see mold spots and know that food is dangerous, or touch fruits and vegetables to judge that it is not fresh.
Chemical signals in the soil become the basis for their perception of whether food is edible, and soil stinkin is just a kind of alarm signal to tell them not to eat the bacteria that emit these signals. A new study in Applied and Environmental Microbiology has found that a Gram-positive bacterium called Streptomyces coelicolor emits earthy tyroxins, but curiously, this molecule does not have any effect on the growth of bacteria.
Earth stinkin appears to be a means for Streptomyces aquama from being eaten by nematodes, as it has no effect on the growth, energy intake, and division of bacteria. "Bacteria don't directly fend off predators, it just seems to be a warning." The lead author of the paper, Professor Brandon Findlay, said.
It is worth mentioning that the study found that soil odorin is harmless to nematodes. The study used a number of Caenorhabditis elegans as test subjects. First, they placed the nematodes on an agar plate with soil taste hormone but no bacteria, and as a result, the nematodes quickly sensed the presence of soil smellin, and then began to move rapidly and frequently changed direction of movement, trying to avoid soil taste.
Subsequently, they artificially mutated the neurons that nematodes use to sense soil odorin, and when placed on an agar plate with the same conditions, the nematodes no longer move around, but the survival of the nematodes is not affected, which shows that the soil odorin itself is harmless to nematodes.
▲Nematodes will avoid soil odorin and avoid eating harmful bacteria (Image source: Reference[2])
Then when Streptomyces blue joined the battle that day, the situation became different. The nematodes of the mutant group do not avoid soil odorin and naturally swallow these bacteria without hesitation. Soon, however, these nematodes died, for only one reason, and Streptomyces azuree was toxic to nematodes.
This is very interesting, bacteria rely on soil stinkin to keep nematodes from preying on themselves, and nematodes rely on soil stink to avoid eating toxic bacteria. This delicate relationship is maintained by the earth odor.
In fact, this example is not the first to be found, for example, previous studies have found that when an nematode becomes ill from eating Pseudomonas aeruginosa, it causes a cascade of reactions that damage the gene called macoilin protein in the nematode. In this way, nematodes can recognize a small molecule of P11 released by Pseudomonas aeruginosa, and when this gene is passed on to offspring, the offspring nematodes avoid the bacteria and no longer prey on them.
Of course, the biological effects of soil odorin are still not fully studied, but it is certain that when we stand outdoors and feel the fragrance of the soil after the rain, there is a big battle full of strategy under your feet...
Resources:
[1] The pleasant smell of wet soil indicates danger to bacteria-eating worms, Concordia researchers find. Retrieved Apr 8th, 2022 from https://www.eurekalert.org/news-releases/948825
[2] The Ubiquitous Soil Terpene Geosmin Acts as a Warning Chemical. Applied and Environmental Microbiology (2022), DOI: 10.1128/aem.00093-22
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