Inactivation of Escherichia coli K12 Using Atmospheric Gas Plasma Produced from Humidified Working Gas

The bactericidal effect of atmospheric gas plasma (AGP) on Escherichia coli K12 cells in 0.9% [w/v] sodium chloride was investigated under normal atmospheric pressure. A plasma‐generating unit was supplied with working gas (air) at 25 °C and a relative humidity (RH) of 0, 14, 32, 43, 50, 60 or 70%. Gas plasma was generated using radio frequency discharge (20 kHz) under atmospheric pressure, and blown onto the surface of an E. coli cell suspension. Seven log‐cycles of cells were completely inactivated within 15 minutes of the start of AGP treatments using working gas at 32, 43, 50 or 60% RH. AGP at 14% RH inactivated only 2 log‐cycles of cells, but no inactivation was observed when air at 0% or 70% RH was used. The inactivation curves were biphasic and the rate constants for both stages were closely related to the RH of the working gas. The rate of cell inactivation was at its maximum at an RH of 43%. The bactericidal effect of the AGP treatment was not a result of ozone generation, however, the moisture content of the working gas was a significant factor in ensuring that E. coli K12 inactivation occurred. The interpretation of these data was that chemical species generated from the water molecules in the working gas were bactericidal in their effects.