Evaluating the roles of OH radicals, H2O2, ORP and pH in the inactivation of yeast cells on a tissue model by surface micro-discharge plasma

Surface micro-discharge (SMD) plasma holds great potential as an effective, economical and safe method for the treatment of diverse skin infections. Although the antimicrobial effect of SMD plasma is undisputed, the factors most responsible for the inactivation effect and their corresponding killing mechanisms are still not fully understood. Herein, we evaluate the roles of hydroxyl radicals (center dot OH), hydrogen peroxide (H2O2), oxidation-reduction potential (ORP) and pH in the inactivation of yeast cells on an agarose tissue model by helium SMD plasma over different irradiation distances and time periods. The center dot OH distribution pattern shows that center dot OH is mostly produced in the center of every hexagon mesh electrode, whose concentration is closely related to humidity and is markedly increased at a shorter irradiation distance and longer treatment time. Meanwhile, the killing pattern of yeast cells corresponds to center dot OH distribution and the inactivation efficiency has a similar trend to that of center dot OH concentration. The results of Pearson correlation analysis reveal that the inactivation efficiency is more dependent on center dot OH and pH compared with H2O2 and ORP. Furthermore, we investigated the effects of center dot OH and pH on yeast cell viability, membrane integrity and intracellular ROS and pH homeostasis by using a specific center dot OH scavenger D-mannitol (D-man) and phosphate buffer solution (PBS). The results showed that D-man could significantly reduce the inactivation efficiency by maintaining cell membrane integrity and intracellular ROS and pH homeostasis, while PBS only slightly mitigates the plasma-caused damage on yeast cells. Based on the results, it is concluded that center dot OH contributes most to the inactivation of yeast cells on a tissue model by helium SMD plasma studied here.