Creature: Medicinal Organisms

What tiny creature makes the earthy smell of rain, grows like a fungus, and has helped scientists fight deadly diseases?

Streptomyces! (Streptomycetaceae)

The Smell That Takes Me Home

I grew up in the northern plains of India, along the banks of the River Ganga, where some of the most fertile agricultural lands in the world have sustained communities for generations. The region is densely populated, and the rhythms of everyday life are shaped by people, agriculture, seasons, and the river itself. Yet amid all that activity, I was fortunate to grow up in a home with a garden.

Some of my fondest childhood memories are of working in that garden with my father. There was something deeply satisfying about getting my hands into the soil, caring for plants, and watching the garden change with the seasons. And then there was the rain.

Anyone who has experienced the Indian monsoon knows that rain is more than a change in weather. After months of intense heat, the first showers bring a certain freshness to the air, a transformation in the landscape, and a fragrance that is almost impossible to describe.

I remember that smell vividly. The rich, earthy aroma rising from the garden as the first drops touched the ground was unlike anything else. It made me feel refreshed, energized, and strangely connected to the world around me.

Thirty years later, it still does.

What I did not realize as a child was that the fragrance I loved, one familiar to countless people around the world, was partly the work of an extraordinary group of microscopic organisms living beneath my feet.

But what gives wet earth its fragrance?

Meet Streptomyces, one of the remarkable inhabitants of the living soil beneath our feet!

Nature’s Invisible Perfumers

At first glance under a microscope, Streptomyces might easily be mistaken for fungi. They grow into networks of branching structures, much like the filaments we associate with mushrooms and molds.

Surprisingly, they are bacteria, belonging to a group called actinobacteria. Unlike many bacteria that exist as individual cells, Streptomyces form branching filaments and produce chains of spores that help them spread through the soil.

And these bacteria are surprisingly colorful! With hundreds of described species, Streptomyces can produce colonies in shades of white, gray, yellow, red, green, blue, and violet. Scientists once relied heavily on the colors of their spore-bearing filaments, known as aerial mycelium, to help identify them. In fact, researchers developed seven color wheels in 1963 specifically for classifying Streptomyces by their spore colors. Today, DNA analysis provides more precise identification, but these colorful colonies remain one of their most fascinating features.

But perhaps their most enchanting characteristic is their ability to produce a chemical compound called geosmin, a name derived from Greek words meaning “earth” and “smell.”

Geosmin contributes to that unmistakable earthy aroma we associate with freshly disturbed soil and rainfall. The broader scent of rain, known as petrichor, is a mixture of compounds, including substances released from plants and soil microorganisms.

When raindrops strike porous soil, tiny air bubbles can form and burst, carrying microscopic droplets and aromatic compounds into the atmosphere. Suddenly, the invisible chemistry of the earth becomes something we can smell.

But this fragrance may serve another purpose entirely. Scientists have discovered that geosmin, along with another compound produced by Streptomyces, attracts tiny soil creatures called springtails, which can help carry the bacteria’s spores to new places. What we experience as the pleasant smell of earth is also part of a microscopic ecological relationship.

There is something wonderfully poetic about this. A microscopic bacterium, invisible to our eyes, produces a fragrance that can travel through the air and awaken memories decades old.

In India, this appreciation of the earth’s fragrance has a history of its own. Traditional perfumers have long captured the scent of rain-soaked earth in mitti attar, a perfume traditionally associated with the aroma of wet clay.

Long before I knew the word geosmin, I knew its fragrance.

A Hidden World Beneath Our Feet

Of course, Streptomyces does much more than give soil its familiar smell.

Imagine a forest floor covered with fallen leaves, twigs, and decaying plant material. To us, this may appear to be nature’s discarded leftovers. To the microscopic communities living beneath the surface, it is an enormous reservoir of nutrients waiting to be transformed.

Streptomyces species are particularly skilled at breaking down complex organic substances, including cellulose from plants and chitin found in fungal cell walls and insect remains. Through decomposition, they help return nutrients to the soil, making them available for other organisms and supporting the continuous cycles of life.

But what happens when the soil dries out? Streptomyces need moisture for active growth, yet many species have an impressive survival strategy. They produce spores that can withstand prolonged dry conditions, remaining dormant until the environment becomes favorable again. These spores are not quite as resistant as those produced by certain other bacteria, but they help Streptomyces persist through changing soil conditions. This makes their relationship with water especially interesting, given how strongly we associate these microbes with the smell of rain.

And they are certainly not confined to fertile gardens. Streptomyces species have been discovered in soils across the world, including forests, agricultural landscapes, and some remarkably harsh environments. Scientists have even isolated species from the hyper-arid Atacama Desert in Chile, one of the driest places on Earth. In 2012, researchers described Streptomyces atacamensis, a species isolated from extremely dry desert soil. Different species have adapted to different conditions, making this genus an important subject for scientists studying microbial diversity and environmental resilience.

They are not working alone.

Healthy soil contains a remarkable community of bacteria, fungi, protozoa, nematodes, earthworms, arthropods, and countless other organisms. Each plays a role in the movement of nutrients, the transformation of organic matter, and the functioning of ecosystems.

A Pharmacy Beneath the Garden

Some members of this genus produce compounds that have transformed modern medicine. One remarkable example is streptomycin, an antibiotic discovered in 1943 at Rutgers University in New Jersey, USA. At the time, researchers were searching for microorganisms capable of producing substances that could fight dangerous bacterial infections, including tuberculosis.

The discovery came through the work of graduate student Albert Schatz in Selman Waksman’s laboratory, with Elizabeth Bugie also contributing to the research. They identified an antibiotic-producing bacterium called Streptomyces griseus; one of the strains came from heavily manured field soil. Streptomycin became the first antibiotic used effectively to treat tuberculosis, offering hope against a disease that had claimed countless lives.

What makes this especially fascinating is that Streptomyces produces such compounds naturally as part of its microbial life. Scientists discovered their medical potential by studying interactions among soil microorganisms, then developing and testing the compounds for therapeutic use.

I find this extraordinary. The same group of bacteria associated with the smell of my childhood garden has also helped change the history of human medicine.

Living Soil, Living Climate

The more I learn about soil, the more I realize that its fertility cannot be understood by looking at minerals and nutrients alone.

Soil is a living system.

Its microscopic inhabitants help break down dead plant material, recycle nutrients, and influence how carbon moves between the soil and atmosphere. Some of that carbon remains in soil organic matter, while some returns to the atmosphere through decomposition. Together with plant roots, fungi, and soil-dwelling animals, these microbial communities help maintain the biological processes on which healthy ecosystems depend.

For ecological restoration, this means that rebuilding fertile soil is not simply a matter of adding nutrients. It also requires restoring the conditions in which diverse soil organisms can thrive.

There is another interesting dimension to this story. When soil microorganisms die, their remains can become part of the soil’s organic matter. Scientists call this microbial necromass, and some of this material can persist in soil when protected through interactions with minerals and soil aggregates. Streptomyces, like other soil bacteria, participates in the wider microbial processes that transform plant-derived carbon. Researchers are increasingly studying how these processes influence whether carbon remains in the soil or returns to the atmosphere. It is an important reminder that soil carbon storage depends not only on plants, but also on what happens within the microscopic community beneath them.

Restoring a degraded landscape requires more than planting trees or spreading seeds. It also means rebuilding the ecological relationships that sustain life below ground. Protecting soils from erosion and excessive disturbance, maintaining plant cover, restoring organic matter, and supporting diverse vegetation can all help create conditions in which soil communities recover.

And as those communities recover, they can support healthier vegetation, nutrient cycling, soil structure, and ecosystem functions that influence water and carbon dynamics.

The connection between soil and climate is complex, but one lesson is clear. We cannot fully restore the living world above ground without caring for the living world below it.

The Quiet Comfort of a Living Forest

There is also something deeply human about our relationship with soil and forests.

For me, the smell of wet earth has always brought a feeling of renewal. Even today, it can transport me back to that garden beside the Ganga, to the presence of my father, and to a time when discovering the world meant getting my hands dirty.

There is growing scientific interest in how time spent in forests affects our well-being. Research on shinrin-yoku, or forest bathing, has found associations with reduced stress, anxiety, and depressive symptoms, although the strength of the evidence varies across studies. These experiences engage many of our senses, from birdsong and filtered sunlight to the familiar fragrance of damp earth.

Forests offer a rich sensory environment. Their sounds, textures, colors, temperatures, and natural fragrances can all contribute to how we experience a place. The earthy aroma of soil is one part of that experience.

It is important, however, to distinguish personal experience from scientific proof. While geosmin is strongly associated with the familiar scent of earth, there is not yet sufficient evidence to conclude that smelling geosmin itself directly improves mental health.

Science explains where the fragrance comes from. My memories explain what it means to me.

The Next Time It Rains

Back then, I knew little about microbial communities, geosmin, carbon cycling, or the intricate relationships that sustain ecosystems. I simply loved being outdoors, working with soil, watching plants grow, and breathing in the fragrance that followed the rain.

Today, I understand a little more about the extraordinary living world responsible for that experience.

And yet, the wonder remains unchanged.

The next time you step into a garden after rainfall, or walk through a forest where the earth is damp and fragrant, take a moment to notice what is happening beneath your feet.

There is an entire world down there, recycling, connecting, transforming, and sustaining life, often without our awareness.

And sometimes, if we are fortunate, that invisible world makes itself known through a familiar scent carried on the breeze.

Thirty years later, the smell of rain still takes me home.

And now I know a little more about whom to thank.

With gratitude for the living soil.

Sources

• Nature Microbiology: Developmentally Regulated Volatiles Geosmin and 2-Methylisoborneol Attract a Soil Arthropod to Streptomyces Bacteria Promoting Spore Dispersal (Becher et al., 2020)
• Emerging Infectious Diseases: Etymologia: Streptomycin (Henry, 2019)
• Nature Communications: Aerosol Generation by Raindrop Impact on Soil (Joung and Buie, 2015)
• CSIRO: The Smell of Rain: How Our Scientists Invented a New Word (Poynton, 2015)
• Journal of Medical Microbiology: Streptomyces From Traditional Medicine: Sources of New Innovations in Antibiotic Discovery (Quinn et al., 2020)
• International Journal of Systematic and Evolutionary Microbiology: Streptomyces atacamensis sp. nov., Isolated From an Extreme Hyper-Arid Soil of the Atacama Desert, Chile (Santhanam et al., 2012)
• Proceedings of the Society for Experimental Biology and Medicine: Streptomycin, a Substance Exhibiting Antibiotic Activity Against Gram-Positive and Gram-Negative Bacteria (Schatz, Bugie and Waksman, 1944)
• International Journal of Mental Health Nursing: The Effects of Forest Bathing on Psychological Well-Being: A Systematic Review and Meta-Analysis (Siah et al., 2023)
• Nature Reviews Microbiology: Life and Death in the Soil Microbiome: How Ecological Processes Influence Biogeochemistry (Sokol et al., 2022)
• Applied Microbiology: System of Color Wheels for Streptomycete Taxonomy (Tresner and Backus, 1963)