Inactivation efficacy of plasma-activated water: influence of plasma treatment time, exposure time and bacterial species

This study aimed to evaluate the influence of plasma treatment time, bacterial exposure time to PAW and bacterial species on the inactivation efficacy of plasma-activated water (PAW), with additional investigation of the inactivation mechanisms of PAW. Six bacterial species, includingListeria innocua,Staphyloccus aureus,Escherichia coli,Pseudomonas fluorescens,Shewanella putrefaciensandAeromonas hydrophilawere selected as the representative bacteria. The initial bacterial concentration was around 7 log CFU ml(-1)after mixing with PAW, and the inactivation efficacy was measured after different exposure times during the 4 degrees C storage. Scanning electron microscopy (SEM) images of the bacteria after PAW treatment were carried out to inspect the cell structure damage, and physicochemical properties of PAW, including pH, conductivity and long-living reactive species of H2O2,NO2-, andNO3-, were examined. The results showed that the inactivation efficacy of PAW was positively correlated with plasma treatment time and bacterial exposure time, and for the species examined in this study, the Gram-negative species were more sensitive to PAW than the Gram-positive species. Cell structure damage, including shrinkage, distortion, or holes, was observed after PAW treatment. The pH of PAW was acidified to 2.5-2.9, and conductivity was significantly increased to 518.0 mu s cm(-1).NO2-and H(2)O(2)were reduced during the 48 h storage, while an increased concentration was observed forNO3-. This study demonstrated that the processing parameters of plasma treatment time, exposure time and characteristics of bacteria can significantly affect the inactivation efficacy of PAW.