The earthy smell of a fresh beet may spark delicious thoughts for us, but for a fruit fly, that smell screams danger.
Geosmin, a naturally occurring chemical that gives beets, fresh soil, and corked wine their distinctive smell, is also cranked out by bacteria deadly to fruit flies. And it turns out that the tiny flies have a direct pathway from nose to brain made just to detect that smell and avoid the toxic microbes that produce it.
When Bill Hansson, a professor at the Max Planck Institute for Chemical Ecology in Jena, Germany, and his crew blocked the geosmin-detecting pathway in flies, they ate contaminated food and died. With the pathway intact, they steered clear. The fact that the tiny fly is devoting precious neuronal real estate to this shows how important it has been for survival to be able to detect the smell of spoiled food.
"They have one line for their whole nervous system just telling about this," Hansson tells The Salt. "On their nose they have cells only detecting this. We tested 3,000 compounds and only got a response to geosmin. That's an amazing specificity." His article was published in the journal Cell.
This is the first evidence of a special dedicated pathway for food odor, Hansson says. Animals are also very sensitive to smells that are sexual signals. But those are blends of pheromones, rather than a single smell.
People don't have a dedicated pathway to detect geosmin. That may be because those microbes don't pose a threat to us. Also, humans have many times more smell receptors than fruit flies. And we eat a much broader range of foods. Some of us even like beets.
In fact, geosmin can be a pleasant smell: think about a farmer's field being plowed in the spring, or the garden just after it rains. But it's also responsible for the funky smell that sometimes troubles municipal water systems. (Supertasters may be able to smell germs in their sinuses, according to a recent study, but most of us don't have that talent.)
But the poor grad student who worked with the flies picked up a moldy geosmin stink that followed him around the laboratory. "It really doesn't smell nice," Hansson said. "You can smell him down the corridor."
There's no question that humans and other mammals react strongly to the smell of spoiled food. Just opening the office refrigerator is usually enough to prompt an "ugh!" or sometimes, worse. But unfortunately, people don't have the ability to recognize the smell of dangerous bugs like E. coli or Salmonella.
Still, this research could prove useful to humans. If dedicated smell pathways like the fruit fly's geosmin detector could be found in insects that pose a threat to humans mosquitoes, say, or agricultural pests like the corn borer Hansson thinks they could be used to develop repellents that would be safer than pesticides.
What smell screams danger to Hansson? Not geosmin, but the smell of vomit. Hansson remembers a long car trip in his native Sweden. His young son threw up in the back seat. Hansson found the hours spent smelling even the faintest remains of that smell to be excruciating. "It's such a strong stimulus. And it's one we don't adapt to. It's telling us someone has eaten something that made them sick. That's an important evolutionary signal."