Effect of D-Alanine in Teichoic Acid from the Streptococcus thermophilus Cell Wall on the Barrier-Protection of Intestinal Epithelial Cells

D-Alanylation of teichoic acid (TA) affects various functions of Gram-positive bacteria, including immunomodulatory effects. We investigated in this study the impact of D-alanine (D-Ala) in TA from Streptococcus thermophilus ATCC 19258(T) on the barrier-protecting effect in human intestinal Caco-2 cells. ATCC 19258(T) suppressed the tumor necrosis factor-alpha-induced decrease in transepithelial electrical resistance (TER), an indicator of the barrier function. The D-alanylation of TA in ATCC 19258(T) was growth phase- and culture temperature-dependent. Treatment of ATCC 19258(T) with Mg2+ decreased the dlt mRNA expression and DAla content in TA and also abolished the suppressive effect on the TER decrease. Supplementation with L-alanine (L-Ala) to the broth led to an increase of L-Ala in ATCC 19258(T) and of the intestinal barrier-protecting effect. Taken together, D-Ala in TA played an important role in the barrier-protecting effect of S. thermophilus in the intestinal epithelium, and these beneficial effects could be enhanced by exogenous L-Ala.