Effects of pH on Bacterial Inactivation in Aqueous Solutions due to Low-Temperature Atmospheric Pressure Plasma Application

Low-temperature atmospheric pressure plasmas applied to the surface of an aqueous solution have been shown to be efficient bactericides for bacteria suspended in the solution, if the solution is sufficiently acidic. Especially of interest is the finding that there is a critical pH value of about 4.7 for the bactericidal effects, below which the bacteria are efficiently inactivated and above which the bacteria are hardly affected by the plasma application. It has been also found that the presence of superoxide anion radicals O(2)(-)center dot in the solution is essential for bacterial inactivation by the plasma application. Therefore, the critical pH value may arise from the pK(a) of the equilibrium reaction between O(2)(-)center dot and hydroperoxy radicals HOO center dot, which is known to be approximately 4.8. The present experiments, where plasmas are not directly applied to bacterium surfaces and it has been confirmed that neither UV light nor heat from the plasma is the cause of bacterial inactivation, suggest the importance of highly reactive species generated in the solution via plasma-liquid interaction for the bactericidal effects.