The medicine in the mud
Beneath your feet, in nearly every soil on Earth, lives a genus of bacteria called Streptomyces. These microbes are important to modern medicine. Streptomyces produce over two-thirds of the clinically useful antibiotics of natural origin. This includes well-known drugs like tetracycline, neomycin, and chloramphenicol. They are Gram-positive bacteria with a branching, filamentous structure that resembles fungi, forming a network of hyphae in the soil.
The story of their importance has a specific anchor here in New Jersey. In 1943, at Rutgers University, a team led by Selman Waksman systematically studied soil microbes. This research led to the discovery of streptomycin, isolated from the species Streptomyces griseus. The discovery is credited to Waksman's graduate student, Albert Schatz. Streptomycin was the first effective treatment for tuberculosis, a disease that was a leading cause of death at the time. For this work, Waksman was awarded the Nobel Prize in Physiology or Medicine in 1952. The term "antibiotic" itself was coined by Waksman.
Streptomyces bacteria possess some of the largest genomes in the bacterial world, ranging from 6 to over 12 million base pairs. This large genetic library allows them to produce a vast array of complex secondary metabolites—compounds not essential for basic growth but that provide a competitive advantage, such as killing off rival microbes. A single Streptomyces strain may have the genetic capacity to produce an average of 30 different secondary metabolites.
The smell of spores
The familiar earthy smell after a rainstorm, known as petrichor, is largely due to a single compound produced by Streptomyces: geosmin. The name comes from the Greek for "earth smell." The bacteria release geosmin when they produce spores, typically when soil begins to dry out. When raindrops hit the dry ground, they trap tiny air bubbles. These bubbles then shoot upwards, bursting from the water droplet as an aerosol that carries the geosmin into the air.
The human nose is exceptionally sensitive to geosmin. It can be detected at concentrations as low as a few parts per trillion. This sensitivity is far greater than a shark's ability to detect blood in water.
This scent is useful for the bacteria. Geosmin attracts soil-dwelling arthropods called springtails. These tiny creatures consume the Streptomyces and help disperse their spores, either by carrying them on their bodies or by excreting them in fecal pellets. In this way, the "smell of rain" is an ancient reproductive strategy, ensuring the bacteria can spread to new, more favorable locations.