A novel rhamnose-rich hetero-exopolysaccharide isolated from Lactobacillus paracasei DG activates THP-1 human monocytic cells

Lactobacillus paracasei DG is a bacterial strain with recognized probiotic properties and is used in commercial probiotic products. However, the mechanisms underlying its probiotic properties are mainly unknown. In this study, we tested the hypothesis that the capability of strain DG to interact with the host is, at least partly, associated with its ability to synthesize a surface-associated exopolysaccharide (EPS). Comparative genomics revealed the presence of putative EPS gene clusters in DG genome; accordingly, EPS was isolated from the surface of the bacterium. A sample of the pure EPS from strain DG (DG-EPS), upon NMR and chemical analyses, was shown to be a novel branched hetero-EPS with a repeat unit composed of L-rhamnose, D-galactose, and N-acetyl-D-galactosamine in a ratio of 4:1:1. Subsequently, we demonstrated that the DG-EPS displays immunostimulating properties by enhancing the gene expression of the pro-inflammatory cytokines TNF-alpha and IL-6, and, particularly, the chemokines IL-8 and CCL20 in the THP-1 human monocytic cell line. In contrast, the expression of the cyclooxygenase enzyme COX-2 was not affected. In conclusion, the DG-EPS is a bacterial macromolecule with the potential ability to boost the immune system as either a secreted molecule released from the bacterium or as a capsular envelope on the bacterial cell wall. This study provides additional information about the mechanisms supporting the cross-talk between L. paracasei DG and the host.