Many humans enjoy the taste of bread, beer, soy sauce, and other yeasty treats because they taste so darn good. As it turns out though, we may not be only ones who like these flavors. A report published last week in Nature describes the discovery of bacteria in our guts that survive by off the yeast in our diets.
The researchers noted that many gut bacteria from the phylum bacteroidetes have genomes that contain genes for enzymes that are capable of degrading complex carbohydrates, including one called α-mannan. Curiously, the primary source of α-mannan in the gut is on cell walls of ingested yeasts such as Saccharomyces cerevisiae. The researchers performed a variety of experiments that confirmed that at least one of the bacteroidetes, Bacteroides thetaiotaomicron, could metabolize the yeast cell wall molecule. The scientists also hypothesize that B. thetaiotaomicron evolved this ability as an adaptation to the changing human diet which includes yeasts from of leavened bread and fermented alcohols. The ability to break down and utilize yeast cell wall components as energy gives B. thetaiotaomicron a competitive edge in living in the gut over other bacteria with less metabolic options.
The B. thetaiotaomicron can thrive in the human intestine because of their evolved symbiotic relationship with the human host: the bacteria breaks down the yeast for the human, while at the same time gaining a source of energy. This type of relationship is probably quite common in the gut and likely extends to other popular foods. Who knows, but knowing what we do about the gut-brain axis, maybe these bacteria are actually causing our cravings for bread and beer.