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By Charlie Fehl

Charlie FehlSt. Patrick’s Day has sprung, and a number of us will be supporting our local breweries and Irish taverns tonight (if not a bit earlier). Those who imbibe can rejoice in the knowledge that they are propagating a unique evolutionary niche, which began alongside the advent of fermented grains, though not where we might expect. Rather than residing in human-made fermentation tanks, our own microbiomes have similarly adapted to the world’s oldest beverage. This niche has spread the world over with the consumption of beer, risen bread, and soy sauces.

The microbiome remains a labyrinthine challenge to study, but a growing number of connections to human health and therapeutic microbiome engineering efforts strongly suggest that achieving an understanding of these symbiotic flora and fauna has become all the more important. But back to beer—our microbiome’s ability to digest the complex carbohydrates presented upon yeast digestion was characterized only last year by an international team of glycobiologists led by Harry Gilbert in Newcastle-upon-Tyne.


Interestingly, a particular species, Bacteriodes thetaiotaomicron, has evolved pathways to allow dominance in the gut through digesting a set of complex yeast carbohydrates called α-mannans. B. thetaiotaomicron can utilize various α-mannans as their sole carbon source, but have also maintained alternative routes for glycan utilization. For glycobiologists (and homebrewers) like me, this is very exciting. This yeast glycan-specific pathway suggests that these species indeed involved alongside the evolutionarily recent increase in yeast-fermented foods sometime around 7,000 years ago, give or take a tipsy scribe.

And this is one thirsty microbe—one facet that struck the researchers about this pathway is its stark efficiency. Whereas other microbes can often benefit from partially-digested “seconds” released by other nutrient-metabolizing species, these particular bugs released no detectable sugar byproducts following yeast mannan digestion. This “selfish” model differentiates B. thetaiotaomicron from most other members of the microbiotic gut community. This works both ways for us, though—as the human gut is packed with potential competitors for food, these mannan-digesting specialists have been able to maintain their own evolutionary niche thanks to our bakers and brewers. Since α-mannans can sensitize immunoinflammatory responses, even leading to Crohn’s disease, these bacteria helpfully remove a potential allergen from our systems.

Brewing Process

So, can altering your intake of yeasty foods (beer, bread, and wine) help tune the “right” microbiotic environment to accrue health effects? This remains to be rigorously tested, but connections between a balanced energy homeostasis as well as gut inflammation have been linked to poor microbiome health. These can affect fat storage levels and levels of glucagon-like peptides, suggesting likely links between the disease etiologies of both obesity and diabetes. The right yeast mannans (or perhaps an unfiltered Belgian beer) could even serve as “prebiotics,” allowing the presence of B. thetaiotaomicron to remain in the gut as sentinels for complex carbohydrate allergens.

So, be sure to enjoy your St. Patrick’s Day with either beer or risen bread. Sadly, traditional Irish soda bread won’t count in this round. Now you can share the knowledge with fellow imbibers that some of evolution’s youngest joiners to the human metagenome will certainly be raising their own versions of pint glasses to you (consisting of a complex network of α-1.6-mannosidases, presumably). This truly, then, is a toast to good health. Sláinte!

Charlie Fehl is a postdoc at the University of Oxford, where he studies carbohydrate biochemistry and biocatalysis with Benjamin G. Davis. He is interested in chemical biology tool development for pharmaceutical and biomedical applications.