In their latest paper in JPhysD, authors from the Technology Research Institute of Osaka Prefecture and Osaka University, Japan report their findings that plasma-treated water (PTW) exhibits strong bactericidal activity against Escherichia coli in suspension, even within a few minutes of preparation:
PTW can be easily prepared; distilled water is directly exposed to low-temperature atmospheric pressure helium plasma.
The bactericidal activity of PTW decays by first order kinetics. Its half-life is 2.3 minutes at room temperature and gets longer at lower temperature; 2.9 days at home freezer temperatures (-18ºC) and 3 weeks at -30ºC. In contrast, half-life is shorter at higher temperatures; 7.9 seconds at human body temperature (37ºC). It means that PTW can rapidly inactivate bacteria and mitigate harmful effects to human tissue when used for disinfection of the human body. Besides, we can preserve bactericidal PTW at low temperature until use.
The bacterial inactivation by PTW is caused by superoxide anion radical due to superoxide dismutase (SOD) assay and electron spin resonance (ESR) analysis. PTW with SOD does not show bactericidal activity, while PTW with bovine serum albumin (BSA, non-enzymatic protein) instead of SOD shows a strong bactericidal effect. ESR analysis of PTW using a spin trapping reagent shows a clear spectrum to catch superoxide anion radical instead of the other radicals like the hydroxyl radical.
Surprisingly, nitrogen is indispensable to prepare PTW (oxygen is not necessary)! We controlled ambient and dissolved gas species using a gastight chamber for preparation of PTW and studied superoxide formation and bactericidal effect. The results shown in figure 1 above (Figure 9 in the paper) indicate intense ESR signal of superoxide adduct and strong bactericidal activity if nitrogen is involved in ambient and dissolved gases.
Molecular nitrogen is required both in the ambient gas and in the distilled water to prepare bactericidal PTW. The reactive species in PTW is not a common active species like hydrogen peroxide or ozone. Moreover, peroxynitrous acid has a smaller activation energy and is not stable at low pH. Peroxynitic acid is only agreed with the properites of thermal stability and superoxide production in PTW. Therefore, we conclude that peroxiytiric acid storeed in PTW induces the bactericidal effect.
About the authors
Satoshi Ikawa received his PhD degree in Agriculture from Osaka Prefecture University, Japan in 2001 by investigation on a receptor protein of insecticidal endotoxin. Now he works as a senior researcher at Technology Research Institute of Osaka Prefecture (TRI-Osaka). His specialities are biochemistry and molecular biology, and recently he investigates on molecular mechanism of plasma sterilization.
Dr Atsushi Tani is a material scientist in Graduate School of Human Development and Environment, Kobe University, Japan (previously in the Graduate School of Science, Osaka University, Japan). His research focuses on basic science of electron spin resonance as well as the application to earth and planetary science.
Dr Yoichi Nakashima is a senior researcher in Technology Research Institute of Osaka Prefecture (TRI-Osaka). His specialities are analytical chemistry and environmental chemistry, and recently he is interested in a chemical analysis of reactive oxygen and nitrogen species (RONS).
Dr Katsuhisa Kitano is a plasma scientist at Graduate School of Engineering, Osaka University, Japan. He collaborates with many researchers from various fields for interdisciplinary research of plasma medicine.
This work is licensed under a Creative Commons Attribution 3.0 Unported License. Figure taken from Satoshi Ikawa et al 2016 J. Phys. D: Appl. Phys. 49 425401. © IOP Publishing, All Rights Reserved.
Categories: Journal of Physics D: Applied Physics