Physiology and antioxidant activity of Pediococcus pentosaceus R1 and Lactobacillus fermentum R6 in response to lactic acid stress

The effect of lactic acid stress on the physiology and antioxidant activity of Pediococcus pentosaceus R1 and Lactobacillus fermentum R6 was investigated in this study. Both the strains were incubated in de Man, Rogosa, and Sharpe (MRS) broth with different pH values (6.0, 5.5, 5.0, 4.5, 4.0, and 3.5) at 37 °C for 1 h. The results revealed that the survival rates of both the strains significantly decreased with a decrease in pH from 4.5 to 3.5 (P < 0.05). Reactive oxygen species levels increased, while ATP concentrations decreased. Moreover, transmission electron microscopy analysis revealed severe cell damage with a decrease in pH. However, the hydrogen peroxide tolerance, free radical scavenging rates, reducing power, superoxide dismutase and glutathione peroxide activities, and total antioxidant capacities of both the strains were improved with a decrease in pH from 5.0 to 4.0, which indicated that appropriate lactic acid stress can improve the antioxidant activities of both the strains. This study brings a new strategy for improving the antioxidant activities of lactic acid bacteria and provides valuable information on the application of these strains as antioxidant starter cultures in fermented foods.