Neuroscientists have discovered that gut bacteria "speak" with the brain to control food choices in animals. They identified two species of bacteria that have a radical impact on animal dietary decisions.
There's no question that nutrients and the so-called microbiome, the community of bacteria that resides in an animal's gut, including human beings, impact health. For instance, diseases like obesity have been associated with the composition of the diet and the microbiome.
But from there to thinking that mere microbes might be able to control the way we act, that we might not be the absolute masters of our own decisions, is a big step. Or maybe not so big, according to the study published in PLOS Biology by a team from the Champalimaud Centre for the Unknown (CCU), in Lisbon, Portugal, in collaboration with a colleague from Monash University, Australia.
The new study was carried out using the fruit fly, which allowed the scientists to dissect the complex interaction of diet, brain, and gut bacteria. They showed that the impact of the fly's gut bacteria is so profound that, when the flies were fed a diet lacking certain essential nutrients, these bacteria stopped them from developing an appetite for those nutrients - and also protected them from the consequences of their absence. The bacteria literally reprogrammed the body's nutritional needs, to the point that they even safeguarded the flies' fertility, which would otherwise have been abolished by the low quality of the diet.
Because natural foods are very complex, the scientists used a synthetic formula to manipulate the fruit fly diet by removing a series of essential amino acids from it. These are the building blocks of proteins that animals are unable to make and therefore must get from their food.
Indeed, the team found that the removal of any single essential amino acid was sufficient to strongly increase the flies' ingestion of protein-rich food. Animals are constantly being faced with different diets of uneven quality. To deal with this challenge, "animals can alter their food choices", says Zita Santos, co-author of the study, which was led by Carlos Ribeiro, principal investigator at the CCU.
The scientists tested the impact on food choices of five species of gut bacteria which are naturally present in fruit flies in the wild. Surprisingly, they found that some flies did not develop an appetite for proteins, and actually continued reproducing in spite of their deficient diet.
The results exceeded expectations: it took just two particular bacterial species to abolish the protein appetite in essential amino acid-deprived animals. "With the right microbiome, fruit flies are able to face these unfavorable nutritional situations", says Santos.
"In the fruit fly, there are five main bacterial species, in humans there are hundreds of them", adds co-author Patrícia Francisco. This highlight, the authors say, the importance of using simple animal models to begin to understand something that may be crucial for human health.
How could the bacteria act on the brain to alter appetite? "Our first hypothesis was that these bacteria might be providing the flies with the missing essential amino acids", Santos replies. "Actually, it may be more complicated than that."
The gut bacteria "seem to induce some metabolic change that acts directly on the brain and the body and mimics a state of protein satiety", adds Santos. In sum, this study not only shows for the first time that gut bacteria act on the brain to alter what animals want to eat, but also that they do so by using a new, unknown mechanism.