https://www.sciencedaily.com/releases/2016/11/161117134626.htm

High-fiber diet keeps gut microbes from eating the colon's lining, protects against infection, animal study shows Date:November 17, 2016Source:University of Michigan Health SystemSummary:

When mice were raised germ-free, then given a transplant of human gut microbes, the impact of fiber on their colons could be seen. Mice fed a high-fiber diet maintained a thick mucus layer along the lining of their colons, while those that received a fiber-free diet saw the mucus layer grow thinner as bacteria capable of digesting mucus proliferated. The thin layer allowed a pathogen bacteria access to the cells of the colon wall.

In a new paper in Cell, an international team of researchers show the impact of fiber deprivation on the guts of specially raised mice. The mice were born and raised with no gut microbes of their own, then received a transplant of 14 bacteria that normally grow in the human gut. Scientists know the full genetic signature of each one, making it possible to track their activity over time. Advanced genetic techniques allowed the scientists to determine which bacteria were present and active under different fiber content — and those with no fiber. They also infected some of the mice with a bacterial strain that does to mice what certain strains of Escherichia coli can do to humans — cause gut infections that lead to irritation, inflammation, diarrhea and more. The mucus layer stayed thick, and the infection didn't take full hold, in mice that received a diet that was about 15 percent fiber from minimally processed grains and plants. But when the researchers substituted a diet with no fiber in it, even for a few days, some of the microbes in their guts began to munch on the mucus. They also tried a diet that was rich in prebiotic fiber — purified forms of soluble fiber similar to what some processed foods and supplements currently contain. This diet resulted in the same erosion of the mucus layer as observed in the lack of fiber. The researchers also saw that the mix of bacteria changed depending on what the mice were being fed, even day by day. Some species of bacteria in the transplanted microbiome were more common — meaning they had reproduced more — in low-fiber conditions, others in high-fiber conditions. And the four bacteria strains that flourished most in low-fiber and no-fiber conditions were the only ones that make enzymes that are capable of breaking down the long molecules called glycoproteins that make up the mucus layer. In addition to looking at the of bacteria based on genetic information, the researchers could see which fiber-digesting enzymes the bacteria were making. They detected more than 1,600 different enzymes capable of degrading carbohydrates — similar to the complexity in the normal human gut. Just like the mix of bacteria, the mix of enzymes changed depending on what the mice were being fed, with even occasional fiber deprivation leading to more production of mucus-degrading enzymes. Images of the mucus layer, and the "goblet" cells of the colon wall that produce the mucus constantly, showed the layer was thinner the less fiber the mice received. While mucus is constantly being produced and degraded in a normal gut, the change in bacteria activity under the lowest-fiber conditions meant that the pace of eating was faster than the pace of production — almost like an overzealous harvesting of trees outpacing the planting of new ones. When the researchers infected the mice with Citrobacter rodentium — the E. coli-like bacteria — they observed that these dangerous bacteria flourished more in the guts of mice fed a fiber-free diet. Many of those mice began to show signs of illness and lost weight. When the scientists looked at samples of their gut tissue, they saw not only a much thinner or even patchy mucus later — they also saw inflammation across a wide area. Mice that had received a fiber-rich diet before being infected also had some inflammation but across a much smaller area. Your diet directly influences your microbiota, and from there it may influence the status of your gut's mucus layer and tendency toward disease. But it's an open question of whether we can cure our cultural lack of fiber with something more purified and easy to ingest than a lot of broccoli."

Journal Reference:

1. Mahesh S. Desai et al. A Dietary Fiber-Deprived Gut Microbiota Degrades the Colonic Mucus Barrier and Enhances Pathogen Susceptibility. Cell, November 2016 DOI: 10.1016/j.cell.2016.10.043

University of Michigan Health System. "High-fiber diet keeps gut microbes from eating the colon's lining, protects against infection, animal study shows." ScienceDaily. ScienceDaily, 17 November 2016. <www.sciencedaily.com/releases/2016/11/161117134626.htm>.

• •Low-fiber diet promotes expansion and activity of colonic mucus-degrading bacteria
• •Purified prebiotic fibers do not alleviate degradation of the mucus layer
• •Fiber-deprived gut microbiota promotes aggressive colitis by an enteric pathogen

Despite the accepted health benefits of consuming dietary fiber, little is known about the mechanisms by which fiber deprivation impacts the gut microbiota and alters disease risk. Mice were colonized with a synthetic human gut microbiota composed of fully sequenced commensal bacteria Thr colonic mucus barrier serves as a primary defense against enteric pathogens. During chronic or intermittent dietary fiber deficiency, the gut microbiota resorts to host-secreted mucus glycoproteins as a nutrient source, leading to erosion of the colonic mucus barrier. Dietary fiber deprivation, together with a fiber-deprived, mucus-eroding microbiota, promotes greater epithelial access and lethal colitis by the mucosal pathogen, Citrobacter rodentium. The study reveals intricate pathways linking diet, the gut microbiome, and intestinal barrier dysfunction.