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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 (•OH), hydrogen peroxide (H 2 O 2 ), 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 •OH distribution pattern shows that •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 •OH distribution and the inactivation efficiency has a similar trend to that of •OH concentration. The results of Pearson correlation analysis reveal that the inactivation efficiency is more dependent on •OH and pH compared with H 2 O 2 and ORP. Furthermore, we investigated the effects of •OH and pH on yeast cell viability, membrane integrity and intracellular ROS and pH homeostasis by using a specific •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 •OH contributes most to the inactivation of yeast cells on a tissue model by helium SMD plasma studied here.

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