Bees can synthesize specific nutrients that support the colonization of their intestinal bacteria. A specific bacterium in the intestine (“Snodgrassella alvi”) which cannot metabolize the sugar necessary to grow, manages to colonize the intestine of bees when no other bacteria are yet present; this happens thanks to the symbiotic relationship that is established between the host and the intestinal microbiota of the bees.
Gut bacteria play an important role for their host: they provide energy by degrading indigestible food, train and regulate the immune system, protect against the invasion of pathogenic bacteria, and synthesize neuroactive molecules that regulate their host’s behavior and cognition.
Snodgrassella alvi is a species-specific bacterium that lives exclusively within a limited section of the digestive canal of bees and is essential for insects to digest honey and pollen: it has co-evolved with bees and has established a symbiotic relationship helping these insects not only in the nutritional process but also by regulating the functioning of the immune system. It is a fundamental bacterium for the survival of bees (it can represent up to 40% of the intestinal microflora) but is also found in many species of honeybees, stingless bees, and bumblebees.
By measuring metabolites in the gut, it was discovered that the bee synthesizes several acids (including citric, malic and 3-hydroxy-3methylglutaric acid) which are exported into the gut: these acids are less abundant when bacteria are present. To demonstrate that the bee directly allows S. alvi to colonize its gut by providing the necessary nutrients, the microbiota-free bees were given a special glucose diet, in which the natural 12C carbon atoms in glucose were replaced with the isotopes labeled 13C. Once the bees were colonized with S.alvi, the two-dimensional image of the atoms in the bee’s gut showed that the S.alvi cells were significantly enriched in 13C: bees metabolize the sugars in their diet and convert them into organic acids that are excreted into the intestine, where the native microbiota uses them as energy. This close metabolic link between the host and its microbes may explain why bees have such a distinct, specific, and stable microbiome.
At the same time, with these discoveries, it becomes necessary to re-evaluate the impact of stress factors on bees and identify the interactions between pesticides and their entire intestinal microbiota; changes in their habitat endanger the delicate symbiotic relationship between host and intestinal microbiota. Stress from emerging pathogens and pesticides has already been linked to gut dysbiosis and changes in foraging habits but could potentially also alter the link between our largest pollinators and food production.