http://www.medicalnewstoday.com/articles/287738.php

Beer and bread have been staples of the human diet for over 7,000 years. Over that period, a bacterium has evolved almost exclusively within the human gut that is able to eat its way through the yeast in these food products. A new study now suggests this bacterium may provide the basis for new treatments for yeast infections and autoimmune diseases, such as Crohn's disease.

Carbohydrates derived from the yeast cell wall are called mannans and are usually found in fermented foods such as bread, beer, wine and soy sauce.
The study - led by Newcastle University in the UK and the University of Michigan Medical School, and published in the journal Nature - set out to identify the complex mechanism by which the common bacteria found in the human gut is able to target and feast upon yeast carbohydrates.

These carbohydrates, derived from the yeast cell wall, are called mannans and are usually found in fermented foods such as bread, beer, wine and soy sauce. However, research has suggested that, in some cases, mannans are harmful to human health.

Previously, scientists had believed that the mechanism behind the breaking down of mannans was related to the "recycling" ability of common gut bacteria. In this process, cells hosting carbohydrates are constantly shed and renewed, which keeps the intestinal lining healthy.

Instead, the researchers found it is just one bacterium specific to the human gut - Bacteroides thetaiotomicron - that is responsible for hunting out and degrading the yeasts we consume from our bread and beer.

Co-author Eric Martens, PhD, of the University of Michigan's Department of Microbiology and Immunology, says that one of the big surprises in the study - which involved feeding laboratory mice a diet consisting of 50% yeast leavened bread - was how B. thetaiotaomicron is sensitively "tuned" to recognize the complex yeast carbohydrates found in beer, wine and bread.

In their study, the authors say that the omnivorous diet of the host requires the bacteria comprising the microbiota in the gut to rapidly adapt to the available nutrients. Therefore, while many organisms in the microbiota produce enzymes that attack the major components of the human diet - such as starch - bacteria such as B. thetaiotaomicron may have thrived by targeting the "low-abundance, highly complex dietary glycans" that most organisms are unable to metabolize.

The yeast we consume 'impacts physiology of gut bacteria'
"Even the relatively small amounts of yeast that we commonly consume in foods are enough to impact the physiology of our friendly gut bacteria," Martens says.

Martens and colleagues believe their identification of the B. thetaiotaomicron yeast-degrading process could accelerate development of prebiotic medicines to help people with bowel problems or autoimmune diseases. This is because glycans - sugars that can contain mannans - have been implicated in autoimmune diseases, such as Crohn's disease.

Co-author Harry Gilbert, PhD, professor of biochemistry at Newcastle University, explains:

"People are very interested in developing dietary regimes where good bacteria are of benefit. When you have certain bacteria dominant in the gut these microorganisms can produce molecules which have health-promoting effects.

The more you understand about how complex glycans are degraded, the more you can think about developing sophisticated prebiotics that target the growth of specific beneficial bacteria."