Capsular polysaccharide inhibits adhesion of Bifidobacterium longum 105-A to enterocyte-like Caco-2 cells and phagocytosis by macrophages

Abstract

Background Bifidobacterium longum 105-A produces markedly high amounts of capsular polysaccharides (CPS) and exopolysaccharides (EPS) that should play distinct roles in bacterial–host interactions. To identify the biological function of B. longum 105-A CPS/EPS, we carried out an informatics survey of the genome and identified the EPS-encoding genetic locus of B. longum 105-A that is responsible for the production of CPS/EPS. The role of CPS/EPS in the adaptation to gut tract environment and bacteria-gut cell interactions was investigated using the ΔcpsD mutant.ResultsA putative B. longum 105-A CPS/EPS gene cluster was shown to consist of 24 putative genes encoding a priming glycosyltransferase (cpsD), 7 glycosyltransferases, 4 CPS/EPS synthesis machinery proteins, and 3 dTDP-L-rhamnose synthesis enzymes. These enzymes should form a complex system that is involved in the biogenesis of CPS and/or EPS. To confirm this, we constructed a knockout mutant (ΔcpsD) by a double cross-over homologous recombination. Compared to wild-type, the ∆cpsD mutant showed a similar growth rate. However, it showed quicker sedimentation and formation of cell clusters in liquid culture. EPS was secreted by the ∆cpsD mutant, but had altered monosaccharide composition and molecular weight. Comparison of the morphology of B. longum 105-A wild-type and ∆cpsD by negative staining in light and electron microscopy revealed that the formation of fimbriae is drastically enhanced in the ∆cpsD mutant while the B. longum 105-A wild-type was coated by a thick capsule. The fimbriae expression in the ∆cpsD was closely associated with the disappearance of the CPS layer. The wild-type showed low pH tolerance, adaptation, and bile salt tolerance, but the ∆cpsD mutant had lost this survivability in gastric and duodenal environments. The ∆cpsD mutant was extensively able to bind to the human colon carcinoma Caco-2 cell line and was phagocytosed by murine macrophage RAW 264.7, whereas the wild-type did not bind to epithelial cells and totally resisted internalization by macrophages.ConclusionsOur results suggest that CPS/EPS production and fimbriae formation are negatively correlated and play key roles in the survival, attachment, and colonization of B. longum 105-A in the gut.Electronic supplementary materialThe online version of this article (doi:10.1186/s13099-017-0177-x) contains supplementary material, which is available to authorized users.

Immunomodulatory Roles of Polysaccharide Capsules in the Intestine

“…More studies are needed expand the known lexicon of bacterial polysaccharide-immune system interactions, and they may even lead to the discovery of new, bioactive CPS for potential use as therapeutics to improve human health. (13,26,29,30,32,34,41,45,49,53).…”

“…In addition, innate immune phagocytes cultured with B. thetaiotaomicron expressing antistimulatory CPSs were poor antigen presenters to the adaptive immune system. Bifidobacterium longum's EPS also inhibited macrophage phagocytosis (32). In contrast, other MPGs can promote the maturation of innate immune cells so that they serve as better antigen presenting cells (APCs) to the adaptive immune system.…”

Abundance and co-occurrence of extracellular capsules increase environmental breadth: Implications for the emergence of pathogens

“…To colonize the gut, the bacteria have to first withstand the harsh conditions of the stomach and then grow and multiply in the duodenum and colon, in the presence of bile salts. In Bifidobacterium longum , capsule expression would enhance survival in the stomach and allow growth under high concentrations of detergent-like bile salts in the duodenum [ 65 ]. Similarly, a study performed in yeast has shown that although capsules from environmental and pathogenic strains display similar composition and features, they fulfil different roles [ 66 ].…”

Interactions of Surface Exopolysaccharides From Bifidobacterium and Lactobacillus Within the Intestinal Environment

“…It is worth noting that the function of these genes has been mostly proposed by means of in silico analyses after homology comparison with the information held in genetic databases (Hidalgo-Cantabrana et al, 2014 ; Ferrario et al, 2016 ; Jiang and Yang, 2018 ). For only a few genes has the function been demonstrated by means of functional assays; examples of functionally characterized eps genes include the GTF (Lamothe et al, 2002 ) also the priming-GTF (Lebeer et al, 2009 ; Fanning et al, 2012 ; Tahoun et al, 2017 ), chain-length determining genes (Bouazzaoui and LaPointe, 2006 ; Hidalgo-Cantabrana et al, 2015 ), and transcriptional regulators of the eps operons (Dertli et al, 2016 ).…”