Olive oil polyphenol extract inhibits vegetative cells of Bacillus cereus isolated from raw milk

This study was conducted to analyze the antibacterial effect of olive oil polyphenol extract (OOPE) against vegetative cells of Bacillus cereus isolated from raw milk and reveal the possible antibacterial mechanism. The diameter of inhibition zone, minimum inhibitory concentration, minimum bactericidal concentration, and survival counts of bacterial cells in sterile normal saline and pasteurized milk were used to evaluate the antibacterial activity of OOPE against B. cereus vegetative cells. The changes in intracellular ATP concentration, cell membrane potential, content of bacterial protein, and cell morphology were analyzed to reveal possible mechanisms of action. Our results showed the diameter of inhibition zone, minimum inhibitory concentration, and minimum bactericidal concentration of OOPE against B. cereus vegetative cells were 18.44 ± 0.55 mm, 0.625 mg/mL, and 1.25 mg/mL, respectively. Bacillus cereus GF-1 vegetative cells were decreased to undetectable levels from about 8 log cfu/mL after treatments with 0.625 mg/mL of OOPE in normal saline at 30°C for 3 h and in pasteurized milk at 30°C for 10 h. The antibacterial mechanisms of OOPE against B. cereus GF-1 vegetative cells may be due to the reduction of intracellular ATP concentrations, cell membrane depolarization, decrease of bacterial protein content, and leakage from cytoplasm. These findings illustrated that OOPE could be used to prevent the growth of contaminating B. cereus cells in dairy products.