[Synergistic inactivation of Bacillus subtilis by high hydrostatic pressure and Nisin at neutral pH]

High hydrostatic pressure (HHP) could be synergized with Nisin in neutral food system to achieve commercial sterilization. The mechanism of synergistic inactivation by HHP treatment and Nisin was investigated at molecular level alongside ultramicroscopic structure.

Bacillus subtilis strain was subjected to HHP (100 -500 MPa for 15 min) alone and in combination with Nisin (200 IU/mL) in PBS (pH 7.0). Membrane permeability was studied by fluorescent dye and ultraviolet absorption. Membrane lipid, protein and DNA changes were detected by Fourier Transform Infrared. Transmission Electron Microscope (TEM) was used to detect the morphological changes.

In neutral food system, the combined effect of HHP and Nisin reduced the population of the bacteria by 2.5 log, while at the same time increasing the membrane permeability by 10% - 60%. The physical state of membrane phospholipid molecules changed from liquid crystalline to gel state with a decrease in membrane fluidity. Treatments at elevated pressures were observed to cause protein denaturalization. Results from TEM demonstrate that cell content leaked owing to the synergistic effect.

This confirms that Nisin is an effective bactericide when combined with HHP in neutral food and that the two inactivation methods have a synergistic effect.