Carrageenan oligosaccharides and associated carrageenan-degrading bacteria induce intestinal inflammation in germ-free mice

Carrageenans (CGNs) are widely used in foods and pharmaceuticals although their safety remains controversial. To investigate the effects of CGNs and CGN-degrading bacteria in the human colon, we screened for CGN degradation by human fecal microbiota, and for inflammatory response to CGNs and/or CGN-degrading bacteria in germ free mice. Thin-layer chromatography indicated that high molecular weight (MW) CGNs (≥100 kDa) remained undegraded in the presence of human fecal microbiota, whereas low MW CGNs, i.e., κ-carrageenan oligosaccharides (KCO, ~4.5 kDa) were degraded when exposed to seven of eight human fecal samples, although sulfate groups were not removed during degradation. Bacteroides xylanisolvens and Escherichia coli isolates from fecal samples apparently degraded KCO synergistically, with B. xylanisolvens serving as the primary degrader. Combined treatment of KCO with KCO-degrading bacteria led to greater pro-inflammatory effects in the colon and rectum of germ-free mice than either KCO or bacteria alone. Similarly, p-p38-, CD3-, and CD79a-positive immune cells were more abundant in combined treatment group mice than in either single treatment group. Our study shows that KCO-degrading bacteria and the low MW products of KCO can promote proinflammatory effects in mice, and represent two key markers for evaluating CGN safety in foods or medicines.