Earthy funk lures tiny creatures to eat and spread bacterial spores

The master chemists known as Streptomyces bacteria have turned a compound rich with the tangy odor of moist soil into a hitchhiking scam.

This group of bacteria, the inspiration for streptomycin and other antibiotics, can release a strong, earthy whiff of what’s called geosmin. It’s not just an everyday scent for them. Some bacterial genes that regulate spore-making also can trigger geosmin production, an international research team reports April 6 in Nature Microbiology. When bacteria start making spores, geosmin wafts into the soil and attracts hungry little arthropods called springtails. They feast on the bacteria, inadvertently picking up spores that hitchhike to new territory, says Klas Flärdh, a microbiologist at Lund University in Sweden.

Geosmin floats off many environmental microbes, including virtually all Streptomyces. People as well as many other animals can detect low concentrations of it. For instance, the common Drosophila lab fruit fly dedicates a circuit in its sensory wiring just to detecting geosmin, which the flies find repellant. That kind of disgust might help animals avoid microbially contaminated food. Various springtails, however, flock to the smell.

Springtails abound in soil (SN: 1/19/14). The “spring” part of their name comes from a prong latched against the body that snaps loose to smack the ground in a crisis, bouncing the springtail up and away from danger.

Scuttling specks of springtails showed up in unusual numbers when coauthor Paul Becher set out bits of Streptomyces bacteria forming spores under shrubbery at the Swedish University of Agricultural Sciences in Alnarp. A springtail can smell the bacterial geosmin, Becher, Flärdh and colleagues say after testing the antenna sensitivity of a pale, all-female kind popular in labs, Folsomia candida.

Genetics linked the alluring geosmin odor to the bacterial phase of making spores. During that phase, a Streptomyces’ usual thready network starts pushing up spore-making structures. “Like skyscrapers,” says Flärdh with a microbiologist’s sense of “tall.” Lab F. candida springtails readily grazed on these micro skyscrapers, and tests confirmed that spores from the bacteria stuck to the springtail bodies. Spores can also spread from fecal pellets.

The idea of feasting springtails that disperse bacterial spores echoes what scientists already know about the little arthropods eating fungi and giving a lift to their spores, even some that are dangerous to other arthropods, says microbiologist Valeria Agamennone, who wasn’t involved in the new research. (She did her dissertation on springtails before joining the Netherlands Organization for Applied Scientific Research in Zeist.) Springtails, she says, may share a long and intimate history with bacteria. She and colleagues have even found some penicillin-making genes that could have originated in bacteria but now mingle with springtail genes.  

The new work on bacterial lures makes “a delightful paper,” says Keith Chater, who worked extensively on Streptomyces at the John Innes Center in Norwich, England, before retiring.  In a long-ago chat with a journalist, Chater off-handedly mused that geosmin from moisture-loving bacteria might let camels sniff their way to water in a desert. “The idea took on a life of its own,” he laments after seeing it repeated more definitively than he meant it. As a bacterial geneticist, he never tested camels. At least now, a somewhat similar geosmin-sniffing tale has turned up with actual data.