Effects of solutions treated with oxygen radicals in neutral pH region on inactivation of microorganism

The inactivation of microorganisms using nonequilbrium atmospheric pressure plasmas has been attracted much attention due to the low temperature processing and high speed treatment. In this study, we have inactivated E. coli suspended in solutions with neutral pH using an atmospheric-pressure oxygen radical source which can selectively supply electrically neutral oxygen radicals. E. coli cells were suspended with deionized distilled water (DDW) (pH$=$6.8) or phosphate buffered saline (PBS) (pH$=$7.4) or Citrate-Na buffer (pH$=$6.5). The treated samples were diluted and spread on nutrient agar (Nutrient Broth). They were cultured at 37$^{\circ}$ C. The inactivation effects of oxygen radicals on those cells in solutions were evaluated by colony-counting method. O2 diluted by Ar gas were employed as a working gas for the radical source. The total gas flow rate and the gas mixture ratio of O2/(Ar$+$ O2) were set at 5 slm and 0.6{\%}, respectively. The distance between the radical exit and the suspension surface were set at 10 mm. As a result, the D values for DDW(pH$=$6.8), PBS(pH$=$7.4) and Citrate-Na buffer(pH$=$6.5) were estimated to be 1.4 min, 0.9min and 16.8 min respectively. The inactivation rates in DDW, PBS were significantly different from that in Citrate-Na buffer.