Cytokine responses of human intestinal epithelial-like Caco-2 cells to the nonpathogenic bacterium Bacillus subtilis (natto)

Intestinal epithelial cells produce cytokines in response to pathogenic bacteria. However, cellular responses of these cells to nonpathogenic strains, such as Bacillus subtilis, are yet to be determined. In this study, we investigate whether epithelial-like human colon carcinoma Caco-2 cells produce cytokines in response to B. subtilis or B. subtilis (natto). The latter strain is utilized for manufacturing the fermented soy food “natto”. Live cells of nonpathogenic B. subtilis JCM 1465T, B. subtilis (natto) and E. coli JCM 1649T, as well as pathogenic S. enteritidis JCM 1652 and P. aeruginosa JCM 5516 strains, induced secretion of interleukin-6 (IL-6) and/or IL-8, but not IL-7, IL-15 or tumor necrosis factor α (TNF-α). Transepithelial electrical resistance (TER) of Caco-2 cell monolayers cultured with E. coli, S. enteritidis or P. aeruginosa decreased more rapidly than that of cells cultured with B. subtilis or B. subtilis (natto). The amounts of cytokine induced by B. subtilis (natto) cells were strain-dependent. Moreover, B. subtilis (natto) cells subjected to hydrochloric acid treatment, but not autoclaving, induced a higher secretion of IL-6 and IL-8 than intact cells. Tyrosine kinase inhibitors, including AG126 and genistein, suppressed cytokine secretion. Our results suggest that the nonpathogenic B. subtilis (natto) bacterium induces cytokine responses in intestinal epithelial cells via activation of an intracellular signaling pathway, such as that of nuclear factor-kappa B (NF-κB).