Researchers from the University of Bonn and Bonn University Hospital elucidate the mechanisms essential for survival
When humans or animals eat something that makes them feel sick, they then avoid that food source. Until now, it was unclear exactly how this avoidance learning occurs. New research shows that communication between brain cells and fat cells may play an important role here.
Participants from the University of Bonn, Tohoku University (Japan), and University Hospital Bonn uncovered a previously unknown mechanism in fruit flies. Similar forms may exist in mammals and humans. The results have now been published in the journal Neuron.
If you’ve ever had an upset stomach after eating a bad meatball, you know how upsetting this experience can be. Within research, this is also known as “conditioned taste aversion.” The brain registers the immune response to the bacteria and its toxins, and concludes from there that the food source should be avoided in the future.
It is not yet known how the discovery of pathogens by the immune system leads to behavioral changes. “This learned food avoidance is found in all species, so we investigated this question in the model organism Drosophila melanogaster,” explains Professor Ilona Grunwald-Cadeau.
Within this model, we can reveal how the brain and body interact to elicit avoidance responses essential for survival. ”
Dr. Ilona Grunwald Cadeau, University of Bonn
Flies initially preferred food contaminated with bacteria
Grunwald Cadeau is the 2nd Director of the Institute of Physiology at the University of Bonn and the University Hospital Bonn. In the current study, her working group is collaborating with researchers at Tohoku University in Japan. Participants had their experimental animals choose between two food sources. One of them was contaminated with the pathogen Pseudomonas entomophila. The other contained a harmless Pseudomonas strain. Otherwise, the two food sources were completely identical.
Flies that have not yet had a bad experience with pathogens prefer harmful baits because they find the smell appealing. “Since this is life-threatening for animals, we wondered how animals would behave if they ingested these bacteria with their food,” explains the scientist. The pathogen did not go undiscovered among flies for a long time. An animal’s innate immune system has sensors that alert it in such cases. “In our experiments, receptors that respond to components of the bacterial cell wall were activated in the body,” explains Grunwald-Cadeau’s colleague Yujie Wang. She conducted most of the experiments as part of her doctoral thesis.
Bacterial sensors lead to behavioral changes
These sensors respond primarily to harmful Pseudomonas strains, but rarely to harmless Pseudomonas strains. Many of them are located on the surface of special neurons near the fly’s throat. These neurons are connected via branches to the fly’s brain as well as to fat stores in the fly’s head. Octopamine, a neurotransmitter closely related to adrenaline, is released in neurons when receptors sound an alarm in the presence of harmful microorganisms. This is transmitted through nerve branches to fat stores.
“Octopamine triggers the formation of another neurotransmitter, dopamine, in fat cells,” says Grunwald-Cadeau. “Dopamine is then transported to the fly’s brain, where it continually increases the activation of neuronal networks important for learning and triggers an avoidance response.” Animals are therefore likely to be deterred from action by the smell of pathogens. “We were able to show that after the flies had experienced spoiled food, they selected a food source containing harmless bacteria,” the scientists explain.
Are hungry flies less picky eaters?
Adipose tissue is significantly involved in this learned behavioral change. But why is this so? “We don’t have a final answer yet,” says Grunwald Kado, a member of the Interdisciplinary Research Area (TRA) Life and Health at the University of Bonn. “However, the flies’ decisions may be related to their nutritional status.”
When an animal is starved, fat cells decrease. When they discover that pathogenic bacteria have been consumed with food, they produce less dopamine accordingly. Perhaps starving animals try to rely on contaminated food sources. “This is a hypothesis that we are currently investigating in further experiments,” explains the scientist.
This result may also be relevant to humans, as our species’ adipose tissue also produces neurotransmitters that act on the brain and influence appetite. Researchers now believe that in eating disorders such as anorexia and obesity, the interactions between the brain, organs, and fat are not working properly. Drosophila allows us to investigate such hypotheses in a simple model organism and understand the underlying mechanisms. This understanding could help influence the complex interactions between metabolism, the immune system, and the brain in disease settings.
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Reference magazines:
Wang, Y. others. (2026) Bidirectional brain-fat body axis for pathogen avoidance. neuron. DOI: 10.1016/j.neuron.2026.03.026. https://www.cell.com/neuron/fulltext/S0896-6273(26)00217-5?_returnURL=.
