We all know that eating toxic foods can be lethal to humans and animals, as it gives bacterial pathogens a way to enter the digestive system and wreak havoc. We can use our sense of smell, however, to identify rotten food. Researchers have recently decoded the neural mechanisms behind a detection system for toxic foods in fruit flies, according to the Max Planck Institute for Chemical Ecology. The neural mechanisms help the fruit flies determine which food is unsafe for consumption and their eggs.
Researchers explain that a highly-sensitive neural line is activated as soon as the smallest amount of geosmin is in the air. Geosmin is a substance released by bacteria and mold fungi that are dangerous to the fly. This signal becomes a warning that screams STOP! at the fly louder than all other signals.
The deaths in Germany in spring 2011 associated with fenugreek sprouts contaminated with EHEC bacteria demonstrated how important it is for humans and animals to be able to identify toxic food before it is consumed. All food has some level of bacteria in it, researchers posit, but how are we able to decide when the concentration of dangerous microbes has become too high?
While in most cases, visual signs allow us to turn down bad food, the best way to avoid such food is to detect certain odors released by hazardous microbes. If a foul-smelling odor is detected, the food is usually thrown away. Researchers, however, wanted to identify the specific neural functions linked to this type of avoidance behavior.
They determined that the vinegar fly (Drosophila melanogaster) and related species were perfect study objects to achieve their goal. When feeding on yeasts growing on rotting fruit, the flies must determine whether the food contains “good” or “bad” microbes.
The researchers discovered that flies eating pathogenic bacteria or fungi were killed and that their eggs did not result in viable larvae. Through experimentation, they found that the fly antennae is more sensitive to geosmin than the human nose, which is highly sensitive to the substance itself. Geosmin produces the odor of wet soil.
“We started with electrophysiological experiments and analyzed all olfactory sensory neurons on the fly antenna successively – more than thousand measurements,” says Marcus Stensmyr, first author of the study.
The researchers found that only a single neuron, labeled “ab4B,” responded to geosmin. These neurons carry the specific receptor Or56a, which researchers found reacts exclusively to geosmin. They also found that only one of the 50 glomeruli labeled DA2 that constitute the antennal lobe, the olfactory center of the flies, was activated by geosmin. Stimulation of DA2 resulted in activation of single type of a specific projection neuron sending a message warning of geosmin presence to higher brain areas. This is the first time that researchers have seen a fully dedicated neural line for an odor involved in feeding behavior.
Researchers used the “Flywalk” system to confirm the nerve and brain measurements in the laboratory. In the “Flywalk” system, single fruit flies are placed into small glass tubes, various odors are released, and the behavior of the flies is recorded by a computer that quantifies the behavior.
Not only does the geosmin stimulus cause the flies to move away from the odor source, it also becomes the most important signal even if other highly attractive odors are competing for their attention. Researchers contend that this is important because odor mixtures are all over the place in a natural environment. The response to geosmin even dictates oviposition behavior in females, meaning that female flies only lay their fertilized eggs on surfaces covered with ordinary yeast.