| Summary: | Alginate has been documented to prevent the development and progression of ulcerative colitis by modulating the gut microbiota. However, the bacterium that may mediate the anti-colitis effect of alginate has not been fully characterized. We hypothesized that alginate-degrading bacteria might play a role here since these bacteria could utilize alginate as a carbon source. To test this hypothesis, we isolated 296 strains of alginate-degrading bacteria from the human gut. <i>Bacteroides xylanisolvens</i> AY11-1 was observed to have the best capability for alginate degradation. The degradation and fermentation of alginate by <i>B. xylanisolvens</i> AY11-1 produced significant amounts of oligosaccharides and short-chain fatty acids. Further studies indicated that <i>B. xylanisolvens</i> AY11-1 could alleviate body weight loss and contraction of colon length, reduce the incidences of bleeding and attenuate mucosal damage in dextran sulfate sodium (DSS)-fed mice. Mechanistically, <i>B. xylanisolvens</i> AY11-1 improved gut dysbiosis and promoted the growth of probiotic bacteria, including <i>Blautia</i> spp. And <i>Prevotellaceae UCG-001</i>, in diseased mice. Additionally, <i>B. xylanisolvens</i> AY11-1 showed no oral toxicity and was well-tolerated in male and female mice. Altogether, we illustrate for the first time an anti-colitis effect of the alginate-degrading bacterium <i>B. xylanisolvens</i> AY11-1. Our study paves the way for the development of <i>B. xylanisolvens</i> AY11-1 as a next-generation probiotic bacterium.
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