Once upon a time… the smell of rain
Once upon a time, there was a scent so familiar that it was given its own name: petrichor.
It is the unmistakable fragrance that rises from the ground after the first drops of a summer rain fall onto dry soil. A comforting, almost universal scent that evokes woodland walks, gardens after a thunderstorm, or cherished childhood memories.
For centuries, people simply believed it was the smell of wet earth. The truth, however, proved far more fascinating.
In the mid-1960s, American researchers H. Gerber and Hubert Lechevalier set out to identify the compounds responsible for this characteristic aroma. Their work led to the discovery of a small organic molecule produced by soil bacteria: geosmin, whose name derives from the Greek words gê (earth) and osmê (smell).
Geosmin possesses a remarkable property: the human nose can detect it at extraordinarily low concentrations—down to just a few parts per trillion. In other words, we are incredibly sensitive to a chemical signal released by microscopic organisms hidden beneath our feet.
The principal producers of geosmin belong to the genus Streptomyces, a group of filamentous bacteria found in soils across the globe.
To microbiologists, Streptomyces are anything but ordinary.
Since the discovery of streptomycin by Selman Waksman and his colleagues in 1943, these bacteria have become renowned as extraordinary natural producers of bioactive compounds. They are responsible for a large proportion of the antibiotics used in modern medicine, as well as numerous antifungal, antiparasitic and other therapeutically valuable molecules.
For decades, geosmin was considered nothing more than a metabolic by-product—a pleasant curiosity of bacterial physiology.
Then, in 2020, a team led by Roman Becher published a study in Nature Microbiology that completely changed this perception.
The researchers demonstrated that geosmin actually serves a far more sophisticated ecological purpose.
As Streptomyces colonies reach maturity, they simultaneously produce spores, ensuring their reproduction, and release geosmin into the surrounding environment. This distinctive scent attracts tiny soil arthropods, particularly springtails, which visit the bacterial colonies and unknowingly leave covered with spores, dispersing them to new habitats.
In other words, these bacteria use their fragrance as a remarkably effective dispersal strategy.
Long before flowering plants evolved perfumes to attract pollinators, soil microorganisms had already developed their own chemical language to travel, colonise new environments and ensure their survival.
The scent we associate with rain is therefore not simply the smell of wet earth.
It is the quiet message left behind by soil bacteria—a fragrance carrying hundreds of millions of years of evolutionary history.
This discovery reminds us that bacteria do far more than survive in their environment. They communicate, interact, and sometimes even influence the behaviour of other organisms through an extraordinary diversity of molecules.
See you soon for the next episode of Smaltis’tory!
Scientific references
• Gerber N.N., Lechevalier H.A. (1965). Geosmin, an earthly-smelling substance isolated from actinomycetes. Applied Microbiology, 13(6), 935–938.
• Becher P.G., Verschut V., Bibb M.J., Bush M.J., Molnár B.P., Barane E., Al-Bassam M.M., Chandra G., Song L., Challis G.L., Buttner M.J., Flärdh K. (2020). Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal. Nature Microbiology, 5, 821–829.