Optimization of the Antimicrobial Effects of Surfactin against Bacillus cereus Spores

The purpose of this study was to establish a three-variable bactericidal model of temperature, time, and concentration to determine the optimal conditions for Bacillus cereus spore inactivation by surfactin. To obtain the binary regression equation of the inactivated spore model, a total of 17 simulations were performed using response surface methodology. The experimental results showed that the three factors each had a discernible but nonequal impact on the inactivation response value. Multiple regression analysis of experimental results using Design-Expert software yielded the following equation: Y = 1.47 + 0.39ξ1(temperature) + 0.38ξ2(time) + 0.39ξ3(concentration) - 0.20ξ1ξ2 + 0.22ξ1ξ2 - 0.12ξ2ξ3 - 0.23ξ12 - 0.11ξ22 - 0.40ξ32. Optimal inactivation of spores was achieved by treatment with surfactin at a concentration of 4 mg/mL for 40 h at 53°C, with the response value reaching 1.8. The spores were treated with surfactin under these conditions; the microstructural changes of spores were observed by use of scanning electron microscopy. We found that the structures of the outer wall of the spores were damaged, whereas the spores in the control sample showed no visible damage.