Carrageenan oligosaccharides and their degrading bacteria induce intestinal inflammation in germ-free mouse

Abstract

Background: Carrageenans (CGNs) are widely used in food and pharmaceutical industries. However, the safety of CGNs is still under debate, because degraded CGNs have been reported to promote an intestinal inflammatory response in animal models. Here, we studied the relationship among CGNs, human gut microbiota, and the host inflammatory response.Methods: TLC was selected for detecting the degradation of KCPs by human gut microbiota in vitro batch fermentation system. PCR-DGGE and real time PCR were used for studying bacterial community. ESI-MS was used for KCPs structure analysis. Hematoxylin-eosin staining (HE), immunohistochemistry (IHC) and RNA-seq were used to evaluated the KCPs on host inflammation response in germ-free mice.Results: Thin-layer chromatography (TLC) data showed that CGNs with a molecular weight (Mw) higher than 100 kDa are not degraded by human fecal microbiota, but low Mw CGNs with an Mw around ~4.5 kDa (KCOs) could be degraded by seven of eight human fecal microbiota samples. KCO degrading B. xylanisolvens was isolated from fecal samples, and PCR-DGGE profiling with band sequencing suggested that B. xylanisolvens was the key KCO degrader in the human gut. Two putative κ-carrageenase genes were identified in the genome sequence of B. xylanisolvens. However, their function on KCO degrading was not verified in vitro. And the sulfate group from KCO is not removed after in vitro degradation by human fecal microbiota, as shown by ESI-MS analysis. The effects of KCO and KCO degrading bacteria on the inflammatory response were investigated in germ-free mice. Increased numbers of P-P38-, CD3a-, and CD79a-positive cells were found in the colon and rectum in mice fed with KCO plus KCO degrading bacteria than in mice fed with only KCO or only B. xylanisolvens and E. coli, as shown by RNA-Seq analysis, HE staining, and IHC. Conclusion: Our data suggested that the presence of KCO degrading bacteria promote the pro-inflammatory effects of CGNs.