Scientists have learned about changes in eating behavior when exposed to bacteria

The bacteria that the body encounters at the beginning of life can influence its behavior after a long time. Scientists have found that early contact with certain bacteria in fruit flies alters the functioning of the taste system: mature individuals begin to avoid food containing a component of the bacterial cell wall. The researchers traced how the signal from intestinal microorganisms is transmitted to the nervous system and why its effect persists after the larva turns into an adult fly. About how bacteria can influence eating behavior throughout life, see the Izvestia article.

Bacteria determine food preferences

The study was conducted on fruit flies and fruit flies. Previously, scientists found that the contact of larvae with certain intestinal bacteria affects the taste reactions of adults. In particular, flies that developed from larvae that interacted with the bacterium Levilactobacillus brevis began to avoid food with peptidoglycan, a substance that is part of the bacterial cell wall. At the same time, another common representative of the intestinal microflora of drosophila, Lactiplantibacillus plantarum, did not cause such an effect. The work was published in the journal PLOS Biology.

In the new work, the researchers tried to find out exactly which bacterial signal triggers this rearrangement. To do this, they used fruit flies grown in conditions that exclude contact with microorganisms, and then added individual bacteria or substances to their environment. It turned out that for the formation of a reaction in adult flies, contact of larvae with live bacteria is necessary: heated microorganisms and the liquid in which they were located did not reproduce the effect.

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The signal turned out to be uracil

Scientists have suggested that uracil, a substance secreted by some bacteria, may play a key role. To test this hypothesis, they used the bacterium Erwinia carotovora Ecc15, also known as Pectobacterium carotovorum. The researchers had strains that differed in their ability to secrete uracil. Larvae that came into contact with bacteria that could produce this substance subsequently formed a peptidoglycan avoidance reaction. Strains with impaired uracil excretion did not produce such a result.

Additional experiments confirmed the significance of the substance itself: the addition of purified uracil to the larval growth medium proved sufficient to cause similar changes. At the same time, other compounds similar in structure, cytosine and thymine, did not reproduce the effect. Thus, scientists have identified uracil as a bacterial signal that triggers changes in eating behavior.

Exposure time also played an important role. Uracil should have been present at an early stage of development: in experiments, it was added to the medium 24 hours after the sterilized eggs were placed. Contact with bacteria in adulthood did not restore the flies' ability to avoid food with peptidoglycan. This indicates the existence of a certain period of development when the body is particularly susceptible to such signals.

From the intestine to the nervous system

Uracil does not directly affect taste neurons. First, it activates the Duox enzyme in intestinal cells, which triggers the formation of reactive oxygen species. Among them is hydrogen peroxide. The researchers found that suppressing the activity of Duox or reducing the level of reactive oxygen species interferes with the formation of a subsequent reaction in adult flies.

Then the signal is perceived by the larval sensory neurons located in the terminal organ, a sensory structure near the oral apparatus. The Gr66a receptor and the TrpA1 ion channel are present in these cells. Scientists have found that a certain type of this channel, TrpA1—B, is needed to start the process. When the researchers disrupted its operation, adult flies lost their ability to avoid food with peptidoglycan.

A sequential chain is obtained: bacteria secrete uracil, the intestine responds to it by producing reactive oxygen species, and sensitive neurons perceive the resulting signal. In the future, it affects how the adult fly reacts to bacterial components. Additional experiments have confirmed that adults actually choose regular food, rather than a mixture with peptidoglycan, if they were exposed to the appropriate effect at an early age.

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How is the signal preserved after metamorphosis

One of the main unresolved questions remains how early experience affects the behavior of an adult fly. During metamorphosis, the nervous system of drosophila is significantly rebuilt, but the consequences of contact of the larva with bacteria persist.

At the same time, different groups of neurons work at different stages of development. In larvae, cells with the Gr66a receptor are needed to trigger changes. In adulthood, the response to peptidoglycan is provided by other sensitive neurons, ppk23-positive. They are the ones who perceive the bacterial component and trigger the avoidance of contaminated food.

The authors of the review consider several possible explanations. Early neuron activity can leave lasting changes in neural circuits that are undergoing metamorphosis. Another option is that exposure to bacteria alters the development of adult sensory systems. So far, the researchers have not established which mechanism ensures the preservation of information, and have not found out whether early contact with microorganisms affects other reactions of ppk23-positive neurons.

It has also not been proven that such a restructuring actually increases the flies' chances of survival or protects them from infections. The authors suggest that early exposure to microbial signals may help the body recognize potential threats in advance, but this still requires a separate check. In addition, the experiments were conducted on fruit flies, so it is impossible to transfer the conclusions to mammals and humans. The work shows one of the possible mechanisms of interaction between the microbiome and the nervous system, but does not prove that intestinal bacteria similarly shape people's food preferences.