What is Geosmin and How Does it Interact with Living Organisms

Geosmin is a naturally occurring compound found in soil, water, and air. It's produced by certain bacteria, such as Actinomycetes, which thrive in these environments.

Geosmin is a key contributor to the earthy or musty smell that's common in soil, lakes, and rivers. This smell is often associated with decaying organic matter.

The name "geosmin" comes from the Greek words "ge" meaning earth and "osmin" meaning smell. It's a fitting name, given its widespread presence in the natural world.

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Geosmin is a smell that's as widespread as it is fascinating, found on every continent, including Antarctica, and even in the peel of beets.

It's produced by a range of bacteria, including Actinobacteria and Myxococcales, as well as fungi and cyanobacteria.

The smell is so potent that we can detect it at a level of 5 parts per trillion, which is incredibly sensitive.

Geosmin is also a common contaminant in drinking water and farmed fish, produced by aquatic equivalents of these soil bacteria.

Its smell is often associated with the earthy scent after a storm, which is why scientists coined the term "petrichor" to describe it.

But geosmin's smell is not just limited to rural areas, it's also found in cities, where it's often masked by other odors.

Interestingly, geosmin repels egg-laying Drosophila melanogaster, which has a dedicated olfactory sensory neuron for geosmin detection.

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Geosmin is produced through a complex process involving multiple genes and enzymes.

For a long time, scientists didn't know how geosmin was made, but the discovery of the genome sequence of Streptomyces coelicolor A3(2) in 2002 shed light on the mystery.

A single gene encodes the enzyme responsible for synthesizing geosmin, as researchers at the John Innes Centre found.

This breakthrough was a significant step forward in understanding the biosynthesis of geosmin.

The biochemistry behind geosmin production was later uncovered by Professor David Cane's lab at Brown University.

Geosmin is a colourless liquid, a member of a large family of Terpenoid oils that have aromatic characteristics, giving them a distinct smell.

These molecules are found in other natural sources, such as eucalyptus, cinnamon, and cloves.

Geosmin has a pungent odour that's extremely sensitive to a range of animals, and humans can detect it in incredibly small amounts - just 100 parts in a trillion.

The production of geosmin in soil is attributed to a single genus of bacteria, called Streptomyces, which are soil-dwelling bacteria that grow spores when faced with unfavourable conditions.

Biosynthesis is the process by which living organisms create complex molecules from simpler ones. This process is crucial for the production of geosmin.

The genome sequence of Streptomyces coelicolor A3(2) was published in 2002, allowing scientists to link genes to their functions and understand the biosynthesis of geosmin.

A single gene encodes the enzyme that catalyzes the synthesis of geosmin, as discovered by Prof Keith Chater's lab at the John Innes Centre.

The biochemistry underlying geosmin production was later discovered by Professor David Cane's lab at Brown University, USA.

Geosmin is produced during growth, with concentrations rising during the exponential phase and peaking during early stationary phase in the bacterium M. xanthus DK1622.

The majority of geosmin is found in the culture medium, suggesting that it provides a fitness benefit to the bacterium during this phase.

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Geosmin is produced by Streptomyces bacteria, specifically to deter predators and attract organisms that disperse spores. This is a clever trick to help the bacteria spread their spores.

Geosmin repels fruit flies, which are less likely to eat materials on which Streptomyces is growing. This means that the bacteria have a better chance of survival.

It's speculated that geosmin also attracts small soil animals that become coated with spores and disperse them through the soil. This is a clever way for the bacteria to get around and spread their presence.

The smell of geosmin is so strong that it can be detected at a level of 5 parts per trillion, which is incredibly trace. This is one of the reasons why we can smell it so well, even in small amounts.

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Geosmin reduces interactions between C. elegans and its producers, making it difficult for the nematodes to graze on certain bacteria.

C. elegans adult hermaphrodites were added to plates containing colonies of S. coelicolor, and the results showed that significantly more worms localized outside the bacterial colonies when geosmin was present.

The presence of geosmin and/or 2-MIB reduced grazing on S. coelicolor, indicating that these compounds play a role in protecting the bacteria from nematode predation.

PR674 worms lacking functional ASE gustatory neurons were found within colonies of all three bacterial strains at all time points, suggesting that the ASE neuron is responsible for bacterial avoidance in C. elegans.

The role of the ASE neuron in bacterial avoidance was not specific to S. coelicolor, as significantly more PR674 adult hermaphrodites were found in the M. xanthus colony than the N2 nematode strain at the 24-h mark.

C. elegans can consume S. coelicolor, and the presence of bacteria in the worm's pharynx can be seen in images of the predation assay.

The presence of geosmin and/or 2-MIB in S. coelicolor colonies reduced the number of worms in the bacterial colonies at the 2-h, 4-h, and 24-h marks.

The addition of geosmin and/or 2-MIB to S. coelicolor J2192 reduced the proportion of worms that appeared dead at the 24-h mark from 37.2% to 10.4%.

Reduced grazing by C. elegans provides a clear fitness benefit to the prey bacteria, making it clear that geosmin plays a role in protecting the bacteria from nematode predation.

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OR11A1 is a receptor that can detect geosmin, a prominent odor quality that's often associated with bacteria.

Geosmin is produced by certain types of bacteria, and OR11A1 is the only human receptor that responds to it.

OR11A1 can detect geosmin at low concentrations, specifically 27.97 ± 7.16 μmol/L, which is a concentration-dependent manner.

This means that OR11A1 can pick up on the presence of geosmin even when it's present in small amounts, making it a useful tool for detecting bacteria.

The OR11A1 receptor is highly specific to geosmin, reacting to it in a way that no other human receptor does.

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The results of the study are quite interesting.

The OR11A1 receptor is capable of detecting geosmin in bacterial extracts at extremely low concentrations, specifically at low nanomolar concentrations.

Typical geosmin-producing bacteria include Streptomyces species, such as S. albus and S. albidoflavus.

We cultured both bacterial strains on BHI agar plates, which led to the discovery that the S. albus extract had a significantly higher geosmin concentration compared to the extract from S. albidoflavus.

The geosmin concentration in the S. albus extract was about 10-fold higher than in the S. albidoflavus extract.

Let's take a closer look at the concentrations of geosmin in the bacterial extracts and the buffer control. The concentrations of geosmin in the extracts were measured in micrograms per liter (μg/L) and nanomoles per liter (nmol/L).

The extracts from S. albidoflavus had a mean concentration of 0.893 μg/L, with a standard deviation of 0.047 and a coefficient of variation (CV) of 5%. This translates to a concentration of 4.9 nmol/L.

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The extracts from S. albus had a mean concentration of 9.53 μg/L, with a standard deviation of 0.49 and a CV of 5%. This translates to a concentration of 52.3 nmol/L.

The buffer control had concentrations of geosmin that were below the detection limit, with values of ≤0.23 μg/L, ≤0.35 μg/L, and ≤0.28 μg/L in the three experiments, respectively. This translates to concentrations of ≤1.9 nmol/L.

Here's a summary of the concentrations in a table:

To study geosmin, researchers used a terpene add-in assay, which is a test that adds a specific chemical to a bacterial colony before introducing C. elegans.

The assay was performed using two types of bacteria: S. coelicolor J2192 and E. coli. Researchers added a chemical called (−)-geosmin to the bacterial colony and allowed it to dry.

The researchers used a negative control, which was autoclaved Nanopure H2O, to compare the results. They evaluated the effects of geosmin on C. elegans ten times for S. coelicolor and three times for E. coli, using a total of 90 adult N2 hermaphrodites for S. coelicolor and 30 for E. coli.

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The smell of geosmin is a subjective experience, and people have different opinions about it. Some find it pleasing, while others consider it unpleasant.

At the John Innes Centre, where there's a long history of Streptomyces research, the smell of geosmin is often described as familiar and pleasing.

Those who enjoy the smell of rain or a walk in the woods might agree that geosmin has a pleasant earthy odor. Parfumiers even add it to fragrances to create an earthy scent.

However, geosmin is also responsible for off-tastes in water, wine, and freshwater fish, making its elimination a priority for some.