Carrageenan-induced colitis is associated with decreased population of anti-inflammatory bacterium, Akkermansia muciniphila, in the gut microbiota of C57BL/6J mice.

Carrageenan as a food additive has been used for years. However, controversy exists regarding to the safety of carrageenan and accumulating evidence indicates that it could induce colitis in experimental models. Here, to provide more information on this issue and solve the debate, we studied and compared in detail the toxic effects of different isomers of carrageenan (κ-, ι-, and λ-) on the colon of C57BL/6J mice. Interestingly, all isomers of carrageenan were found to induce colitis with a comparable activity. Given that carrageenan is unabsorbed after oral administration, and also in light of the fact that gut microbiota plays a pivotal role in the pathogenesis of colitis, we further investigated the effect of carrageenan on gut microbiota using high-throughput sequencing. Intriguingly, carrageenan-induced colitis was observed to be robustly correlated with changes in the composition of gut microbiota. Specifically, all carrageenans significantly decreased the abundance of a potent anti-inflammatory bacterium, Akkermansia muciniphila, in the gut, which is highly relevant for understanding the toxic effect of carrageenan. Altogether, our results corroborate previous studies demonstrating harmful gastrointestinal effect of carrageenan and, from a gut microbiota perspective, shed new light into the mechanism by which carrageenan induces colitis in experimental animals.