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

BACKGROUND

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 105-A CPS/EPS, we carried out an informatics survey of the genome and identified the EPS-encoding genetic locus of 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 Δ mutant.

RESULTS

A putative 105-A CPS/EPS gene cluster was shown to consist of 24 putative genes encoding a priming glycosyltransferase (), 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 (Δ) by a double cross-over homologous recombination. Compared to wild-type, the ∆ mutant showed a similar growth rate. However, it showed quicker sedimentation and formation of cell clusters in liquid culture. EPS was secreted by the ∆ mutant, but had altered monosaccharide composition and molecular weight. Comparison of the morphology of 105-A wild-type and ∆ by negative staining in light and electron microscopy revealed that the formation of fimbriae is drastically enhanced in the ∆ mutant while the 105-A wild-type was coated by a thick capsule. The fimbriae expression in the ∆ was closely associated with the disappearance of the CPS layer. The wild-type showed low pH tolerance, adaptation, and bile salt tolerance, but the ∆ mutant had lost this survivability in gastric and duodenal environments. The ∆ 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.

CONCLUSIONS

Our results suggest that CPS/EPS production and fimbriae formation are negatively correlated and play key roles in the survival, attachment, and colonization of 105-A in the gut.