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Our Own Friendly Neighbourhood Yeast-Breakers

In 2005, scientists analysed the chemical residues inside some ancient Chinese pottery jars and found traces of the alcoholic drink they once contained—a concoction fermented from rice, honey, and fruits. Fermentation means yeast. The jars were 9,000 years old, which means that humans have been using yeast to make drinks for at least that long. First, we brewed booze. Then, we baked bread. And all the while, the bacteria in our guts were adapting.

A team from Newcastle University, led by Harry Gilbert, have shown that a common and well-studied gut bacterium has become exceptionally good at eating the unique carbohydrates found in yeasts. Whenever we eat a sandwich or quaff some beer, this yeast-breaker—Bacteroides thetaiotamicron, or B-theta for short—also gets a rich meal.

Yeasts are fungi. They are surrounded by a sturdy cell wall, which contains a complex group of carbohydrates called alpha-mannans. These consist a few simple sugars that unite, in groups of a hundred or more, to form elaborate structures that look like the head of a gigantic rake—a long backbone, with umpteen prongs branching from it.

These structures are tough work for the meagre set of digestive enzymes encoded within our own genome. But bacteria have no such problems. They are digestive wunderkinds. B-theta alone has over 250 carbohydrate-busting enzymes—one of the largest sets of any microbe. For comparison, we have just 100 or so, even though our genome is 500 times bigger.

Gilbert’s team, including Fiona Cuskin, Elisabeth Lowe, and Max Temple, showed that B-theta’s cutlery set is more than capable of tackling alpha-mannans from yeasts. It uses 15 enzymes for the job. Some pluck the side-chains from the mannans (the prongs of the rake), creating room for others to swoop in and sever the backbone. Slowly, the elaborate molecules are dismantled.

Most of this happens inside the cell itself. After a few initial cuts, B-theta sucks up the fragments of its food and sunders the rest internally. It’s a tidy eater. That was a surprise. “This selfish degradation is at odds with the general thought that carbohydrate breakdown in the gut is a collaborative process,” says Lowe.

There are food chains in your gut that mirror those in, say, the Serengeti. Lions and cheetahs will kill bigger prey, allowing jackals and vultures to pick off the scraps. Similarly, some gut bacteria break down the biggest and hardiest carbohydrates, and release smaller fragments that others can mop up. But B-theta seems to ignore these food chains. It hoards alpha-mannans for itself and devours every last crumb, leaving nothing to scavengers.

“We don’t know why,” says Lowe. But she has an idea. Yeast alpha-mannans are everywhere, not just in our food but also in fungi that naturally grow in our guts. But they’re found at low levels, so they’re not a reliable food source. Lowe’s team suspects that B-theta uses them as a back-up when other nutrients are scarce and given how much effort they take to digest, it’s not going to share any with competitors.

Remember that the gut is an ecosystem, and different gut 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’s a generalist. It’ll tackle most of the foods that we throw at it, which probably explains why it’s one of the most common bacteria in our gut. It can eat, regardless of what we eat.

A few other gut microbes can digest alpha-mannans too. The team found 8 other species with the same ability, although none of them seem to be as accomplished at it 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.

Alpha-mannans are also implicated in some inflammatory bowel diseases like Crohn’s disease. There’s some evidence that people with this condition have antibodies that target mannans and might therefore overreact to the presence of yeast in their guts. They also tend to have less B-theta (and related species), and one company has even licenced a drug called Thetanix—basically, just live B-theta—as a treatment for childhood Crohn’s. “We think our research shows a link as to why Bt is having this anti-inflammatory effect,” says Lowe.

Reference: Cuskin, Lowe, Temple, Zhu, Cameron, Pudlo, Porter, Urs, Thompson, Cartmell, Rogowski, Hamilton, chen, Tolbert, Piens, Bracke, Vervecken, Hakki, Speciale, Munoz-Munoz, Day, Pena, McLean, Suits, Boraston, Atherly, Ziemer, Williams, Davies, Abbott, Martens & Gilbert. 2015. Human gut Bacteroidetes can utilize yeast mannan through a selfish mechanism. Nature http://dx.doi.org/10.1038/nature13995

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