19. 01. 15

Beer and Bread yeast-eating Bacteria Aid Human Health

Bacteria that have evolved to eat their way through yeast in the human intestine could break new ground in the development of new treatments for people suffering from bowel diseases.

Some studies show how microbes in our digestive tract have learned to unravel the difficult to digest complex carbohydrates that make up the yeast cell wall. During 7,000 years of evolution, humans have consumed fermented food and drinks. The ability of strains of Bacteroides thetaiotomicron (or B-theta in short) to degrade the yeast is almost exclusively found in the human intestine. First, we brewed booze. Then, we baked bread. And all meanwhile, the bacteria in our intestines were adapting.

Researchers in Newcastle and Michigan have identified the complex machinery that targets yeast carbohydrates. Yeats are surrounded by a sturdy cell wall, which contains a complex group of carbohydrates called alpha-mannans. These consist of a few simple sugars that unite, in groups of a hundred or more. This has provided a better understanding of how our unique intestinal soup of bacteria -- the microbiome -- has the capacity to obtain nutrients from our highly varied diet.

The structure of yeasts are tough work for our digestive enzymes. However, bacteria have no such problems with these structures. B-theta alone has over 250 carbohydrate-busting enzymes—one of the largest sets of any microbe. In comparison, we have just 100 or so. The research involving scientists from Newcastle, Australia, Canada, USA and Belgium has unraveled the mechanism by which B-theta, a dominant member of the human microbiome, has learned to feast upon difficult to break down complex carbohydrates called yeast mannans. Mannans, derived from the yeast cell wall, are a component in our diet from fermented foods including bread, beer, wine and soy sauce.

"People are very interested in developing dietary regimes where good bacteria are of benefit," explained Gilbert. "When you have certain dominant bacteria in the intestine these microorganisms can produce molecules which have health promoting effects”. The intestine is an ecosystem, and different intestine microbes play different roles within it. There are predators and scavengers, common species and rarer ones, specialists and jacks-of-all-trades. B-theta is the latter. It will tackle most of the foods that we throw at it, which probably explains why it is one of the most common bacteria in our gut.

 “There is a lot of interest in developing prebiotics. The more you understand about how complex carbohydrates are degraded the more you can think about developing sophisticated prebiotics that target the growth of specific beneficial bacteria.” Gideon says. It is hoped the research could aid a better understanding of how to provide nutrients to specific organisms in the microbiome. Gideon who contributed to the work, said: "The ability of B-theta to degrade yeast cell wall components may be of importance in fighting off yeast infections and in autoimmune diseases such as Crohn's disease."

A few other intestine microbes can digest alpha-mannans too. The team found eight other species with the same ability, although none of them seem to be as efficient as B-theta, and most of them are close relatives. This discovery has a few possible implications for human health. As well as digesting Saccharomyces cerevisiae, the yeast that helps us to bake and brew, B-theta can also break down the cell walls of Candida albicans, the fungus that causes thrush and other diseases. Our gut microbes may be able to protect us from this fungus, and the team are looking into this further.

By consuming carbohydrates that we cannot, which they convert to short-chain fatty acids that they secrete into our distal intestine, these bacteria establish a symbiosis that nourishes the cells that line our intestine wall and provide important immune signals that establish a healthy immune response.

Fiona Cuskin, Elisabeth C. Lowe, Max J. Temple, Yanping Zhu, Elizabeth A. Cameron, Nicholas A. Pudlo, Nathan T. Porter, Karthik Urs, Andrew J. Thompson, Alan Cartmell, Artur Rogowski, Brian S. Hamilton, Rui Chen, Thomas J. Tolbert, Kathleen Piens, Debby Bracke, Wouter Vervecken, Zalihe Hakki, Gaetano Speciale, Jose L. Munōz-Munōz, Andrew Day, Maria J. Peña, Richard McLean, Michael D. Suits, Alisdair B. Boraston, Todd Atherly, Cherie J. Ziemer, Spencer J. Williams, Gideon J. Davies, D. Wade Abbott, Eric C. Martens, Harry J. Gilbert. Human gut Bacteroidetes can utilize yeast mannan through a selfish mechanismNature, 2015; 517 (7533): 165 DOI: 10.1038/nature13995

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