Non-thermal plasma at atmospheric pressure has been actively applied to sterilization. However, its efficiency for inactivating microorganisms often varies depending on microbial species and environments surrounding the microorganisms. We investigated the influence of environmental factors (surrounding media) on the efficiency of microbial inactivation by plasma using an eukaryotic model microbe, Saccharomyces cerevisiae, to elucidate the mechanisms for differential efficiency of sterilization by plasma. Yeast cells treated with plasma in water showed the most severe damage in viability and cell morphology as well as damage to membrane lipids, and genomic DNA. Cells in saline were less damaged compared to those in water, and those in YPD (Yeast extract, Peptone, Dextrose) were least impaired. HOG1 mitogen activated protein kinase was activated in cells exposed to plasma in water and saline. Inactivation of yeast cells in water and saline was due to the acidification of the solutions by plasma, but higher survival of yeast cells treated in saline may have resulted from the additional effect related to salt strength. Levels of hydroxyl radical (OH.) produced by plasma were the highest in water and the lowest in YPD. This may have resulted in differential inactivation of yeast cells in water, saline, and YPD by plasma. Taken together, our data suggest that the surrounding media (environment) can crucially affect the outcomes of yeast cell plasma treatment because plasma modulates vital properties of media, and the toxic nature of plasma can also be altered by the surrounding media.

Induction of micronucleus formation (cytogenetic damage) in brain cancer cells upon exposure of dielectric barrier discharge plasma has been investigated. We have investigated the influence of exposure and incubation times on T98G brain cancer cells by using growth kinetic, clonogenic, and micronucleus formation assay. We found that micronucleus formation rate directly depends on the plasma exposure time. It is also shown that colony formation capacity of cells has been inhibited by the treatment of plasma at all doses. Cell death and micronucleus formation are shown to be significantly elevated by 120 and 240 s exposure of dielectric barrier discharge plasma.

The efficacy of seed disinfection utilizing atmospheric pressure non-thermal plasma is still a subject of intensive research. Previously, we found that rice seeds infected by Fusarium fujikuroi (fungus causing bakanae disease) could be disinfected via underwater arc discharge plasma. In this study, we further investigated the mechanism of disinfection and the effect on disease severity. In addition, we evaluated disinfection of rice seeds in air by surface DBD plasma. Disease severity was significantly reduced in seedlings germinated from rice seeds treated with underwater arc plasma, compared to non-treated seeds (44%–62% plasma treated compared to 92% non-treated). A shockwave of 11 atm pressure was generated during arc discharge, which likely caused fungal detachment from the seed surface. Moreover, reactive oxygen species such as atomic oxygen (O) was emitted from the underwater arc discharge plasma and could have contributed to the degeneration of the chemical composition on the seed surface and the inactivation of the fungal spores. Rice seeds treated with surface DBD plasma in the presence of H2O2 at a lower pressure (0.6 atm) for 30 min showed the highest seed disinfection efficiency (about 93% seed disinfection) and a significant reduction in disease severity in the germinated seedlings (23% plasma-treated compared to 100% non-treated). Taken together, our results suggested that underwater arc discharge and surface DBD plasma could be usefully applied to develop control strategies for seed-borne fungal diseases via seed decontamination during dry storage or water imbibition before sowing.

In this study, we generated water and phosphate buffer treated with microwave plasma-generated gas in which the major component was nitric oxide (PGNO), and investigated the efficiency of the treated water and buffer in fertilization and sanitation. Real time NO level monitored by an electrode sensor was linearly increased over PGNO injection time, and removal of O2 from liquid before PGNO injection accelerated NO assimilation into liquids. Residual NO was still present 16 h after PGNO injection was stopped. H2O2, NO2−, and NO3− were also detected in PGNO-treated liquids. Spinach plants applied with 10 and 30 times diluted PGNO-treated water and 0.5 mM phosphate buffer showed slightly higher height and dry weight than control after 5 weeks. Plants grown with 10 and 30 times diluted PGNO-treated water exhibited the increased tolerance to water deficiency. Significant anti-microbial activity within 1 h was observed in un-diluted and in half-diluted PGNO-treated water and 0.5 mM phosphate buffer. Our results suggest that water or phosphate buffer containing NO, H2O2, NO2−, and NO3− can be produced by PGNO treatment, and that PGNO-treated water or buffer can be used as a potential fertilizer enhancing plant vitality with sanitation effect.