In spite of the positive effects of bacteria on health, certain species are harmful, and therefore, animals must weigh nutritional benefits against negative post-ingestion consequences and adapt their behavior accordingly. Here, we use Drosophila to unravel how the immune system communicates with the brain, enabling avoidance of harmful foods. Using two different known fly pathogens, mildly pathogenic Erwinia carotovora (Ecc15) and highly virulent Pseudomonas entomophila (Pe), we analyzed preference behavior in naive flies and after ingestion of either of these pathogens. Although survival assays confirmed the harmful effect of pathogen ingestion, naive flies preferred the odor of either pathogen to air and also to harmless mutant bacteria, suggesting that flies are not innately repelled by these microbes. By contrast, feeding assays showed that, when given a choice between pathogenic and harmless bacteria, flies-after an initial period of indifference-shifted to a preference for the harmless strain, a behavior that lasted for several hours. Flies lacking synaptic output of the mushroom body (MB), the fly's brain center for associative memory formation, lost the ability to distinguish between pathogenic and harmless bacteria, suggesting this to be an adaptive behavior. Interestingly, this behavior relied on the immune receptors PGRP-LC and -LE and their presence in octopaminergic neurons. We postulate a model wherein pathogen ingestion triggers PGRP signaling in octopaminergic neurons, which in turn relay the information about the harmful food source directly or indirectly to the MB, where an appropriate behavioral output is generated.


  • Food and feeding are essential for all animals

  • We show that flies innately prefer the odor of pathogenic bacteria over harmless mutant versions and readily feed on them

  • The Pseudomonas entomophila (Pe) gacA mutant strain is deficient for the GacS/GacA system, which controls the production of secondary metabolites and extracellular enzymes essential for its pathogenicity [13, 19]

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Food and feeding are essential for all animals. Food choices influence the energy budget and the composition of the gut microbiome. Feeding can be dangerous, through toxicity or bacterial infections that follow the ingestion of a spoiled food source. Some foods are innately avoided through the presence of a strongly aversive sensory cue. Toxins or pathogens do not present aversive cues recognizable by external sensory systems, and animals will consume them without expecting any harm or negative consequences. Such negative post-ingestion consequences can help to recognize and avoid food of bad quality and thereby significantly enhance survival chances of the individual and its species


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