Cytoprotective effect of Streptococcus thermophilus against oxidative stress mediated by a novel peroxidase (EfeB)

Streptococcus thermophilus is one of the most important starter species used in the dairy industry and exhibits several beneficial properties for the hosts. However, knowledge of the mechanism of its beneficial effect is still limited. The objective of this study was to investigate the cytoprotective effect of S. thermophilus CGMCC 7.179 with a novel peroxidase (EfeB) against oxidative stress in human intestinal epithelial cells, HT-29. Previously, we identified EfeB in S. thermophilus CGMCC 7.179, which could provide protection when growing at aerobic conditions. Here, we found that, when exposed to 15 mM H2O2, the cell viability of the efeB mutant (ST1314) was much lower than that of strain CGMCC 7.179, and the 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity of strain ST1314 decreased by 15%. When co-incubated with HT-29 cells, strain CGMCC 7.179 stimulated the enhancement of the major antioxidant enzyme activities (superoxide dismutase, glutathione peroxidase, and catalase) in HT-29 cells under 2 mM H2O2-induced oxidative stress, whereas the active decrease of those antioxidant enzymes was observed in strain ST1314. In addition, the intracellular reactive oxygen species content in HT-29 cells co-incubated with strain CGMCC 7.179 was lower than that with strain ST1314 under the same oxidative stress. Furthermore, the protein content of nuclear factor erythroid 2-related factor 2 (Nrf2) in HT-29 cells following strain CGMCC 7.179 treatment was 1.4-fold higher than that with strain ST1314 treatment, and the increased transcription levels of Nrf2-related antioxidant enzyme genes were also observed in strain CGMCC 7.179 cells. All of these results demonstrated that S. thermophilus CGMCC 7.179 enhanced cellular antioxidant responses and endowed host cells with protective effects against oxidative stress mediated by the peroxidase EfeB.