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Iodide Anion could Greatly Promote Bacterial Killing by Helium Plasma Jet
A recent study found low amount of iodide anion could promote the killing of both Gram-negative (P. aeruginosa, E. coli) and Gram-positive (S. aureus) bacteria by helium plasma jet.

This work was done by HUANG Qing and his team in laboratory of irradiation apparatus and physical biology of the institute.  

Due to its great application potentials and unique advantages, cold atmospheric plasma (CAP) emerges as an innovative and attractive sterilization method in bio-medicine field. When contrasted with the conventional sterilization, CAP has many advantages for it can be easily conducted with simple equipment and temperature requirements and so on.

Although several bactericidal factors can be produced by CAP, it is generally deemed that the synergy of reactive oxygen species (ROS) and reactive nitrogen species (RNS) are the main reasons for CAP-induced bacterial inactivation. Using other gases (such as helium, argon, and oxygen), except nitrogen and air, as the plasma working gas can rule out or diminish RNS production, and it also weakens the disinfection capability of CAP for only ROS is produced in these gas plasma systems.

Therefore, an optional method for enhancing the antimicrobial activity of CAP without or with diminishing production of harmful RNS is highly required for practical applications of CAP.

In this work, HUANG’s group found that addition of low amount of iodide anion could promote the killing of both Gram-negative (P. aeruginosa, E. coli) and Gram-positive (S. aureus) bacteria induced by helium plasma jet.

Taking P. aeruginosa for an example, in the absence of iodide, exposure to helium plasma jet for 20s can lead to 5% bacterial inactivation in 1 mL bacterial suspension with density at 107 CFU/mL. While, with the presence of 10 μM, 1 mM and 10 mM iodide, the inactivation ratio increased to 22%, 82% and 100% respectively after the same plasma treatment.

In fact, the enhancement effect of iodine is more significant with the prolongation of plasma treatment time. To explore its mechanism, HUANG’s group compared the inactivation behaviors of bacteria induced by plasma-activated water (PAW) and plasma activated KI (PAI), and by PAW in the absence or presence of KI. They found that the potentiation effect of iodine was due to the oxidation of iodide to active iodine species by the reactive chemical species generated by helium plasma jet.

To further confirm the insiede mechanism, the team adopted several research methods such as addtion of radical scavenger, high performance liquid chromatography and ultraviolet-visible absorption spectroscopy, and they managed to verify that the produced hydrogen peroxide oxidized iodide to triiodide (I3-), which was responsible for the potrntiation effect.

This work provides a basis for the application of CAP in combination with halogen ions for disinfection in the biomedical field.

This work was supported by the National Nature Science Foundation of China and the Natural Science Foundation of Anhui Province.

Left: Schematic diagram of helium plasma jet. Right: Bacterial inactivation induced by helium plasma jet in the presence of differenct concentrations of iodide. (Imaged by KE Zhigang)