Aim: To investigate the ability of Lactic acid bacteria (LAB) to modulate inflammatory reaction in human intestinal cell lines (Caco-2, HT-29 and HCT116). Different strains of LAB isolated from new born infants and fermented milk, together with the strains obtained from culture collections were tested.
Methods: LABs were treated with human intestinal cell lines. ELISA was used to detect IL-8 and TGF-beta protein secretion. Cytokines and Toll like receptors (TLRs) gene expression were assessed using RT-PCR. Conditional medium, sonicated bacteria and UV killed bacteria were used to find the effecter molecules on the bacteria. Carbohydrate oxidation and protein digestion were applied to figure out the molecules' residues. Adhesion assays were further carried out.
Results: It was found that Enterococcus faecalis is the main immune modulator among the LABs by downregulation of IL-8 secretion and upregulation of TGF-beta. Strikingly, the effect was only observed in four strains of E. faecalis out of the 27 isolated and tested. This implies strain dependent immunomodulation in the host. In addition, E. faecalis may regulate inflammatory responses through TLR3, TLR4, TLR9 and TRAF6. Carbohydrates on the bacterial cell surface are involved in both its adhesion to intestinal cells and regulation of inflammatory responses in the host.
Conclusion: These data provide a case for the modulation of intestinal mucosal immunity in which specific strains of E. faecalis have uniquely evolved to maintain colonic homeostasis and regulate inflammatory responses.