The Mechanisms of Action of Cold Atmospheric Air Plasma for Use in Hospital Decontamination

Hospital surfaces are a potential reservoir of infection due to contamination by bacteria. A potential novel method to decontaminate these surfaces is the use of cold atmospheric pressure plasma (CAPP) systems. CAPP has antimicrobial properties yet the mechanisms of action are not fully understood, although it is believed they are partially due, to the production of reactive oxygen and reactive nitrogen species (RONS). As a decontamination tool, CAPP must inactivate not only singular bacteria but also those living within biofilms. Biofilms are multi-cellular, protective structures, which confer a higher resistance to disinfectants than singular, planktonic bacteria. In this project, we aimed to show the potential use of a CAPP jet for hospital decontamination as well as to examine the mechanisms of action of bacterial inactivation.

An atmospheric pressure, air plasma jet was used to inactivated both Grampositive and -negative, clinically relevant bacteria both in planktonic and biofilm phases. These biofilms were grown on common hospital surface materials to reflect real world conditions. The viability of biofilms showed bacterial inactivation after CAPP treatment. CAPP treatment was also examined alongside a normal hospital cleaning agent and in the presence of blood. CAPP treatment inactivated bacteria on hospital materials without hindering normal cleaning.

CAPP can directly inactivate bacteria but also can be used to produce plasma activated liquid (PAL). This liquid, treated by CAPP, also contains antimicrobial properties. PAL produced in this study had increased RONS concentrations but no significant decrease in pH. It was found to inactivate planktonic cells and biofilms as well as being able to reduce contamination of cloths commonly in hospital cleaning. Bacterial inactivation may to be due to RONS in the liquid such as hydrogen peroxide.

The bacterial response to CAPP was also examined. Scanning electron microscopy showed bacterial distortion after CAPP treatment. Damage of the bacterial membrane and penetration of CAPP within biofilm structures was shown by confocal microscopy and quantified through staining techniques. Utilising laser-induced fluorescence, it was found that bacterial inactivation corresponded to the concentration of nitric oxide and atomic oxygen produced by the CAPP jet. Genetic up-regulation of oxidative damage genes in CAPP treated Escherichia coli and Staphylococcus aureus was also shown through quantitative reverse transcription PCR.

In conclusion, we have shown a possible role of CAPP in hospital decontamination through the decontamination of clinically relevant bacteria on common hospital surfaces through both direct and PAL treatment. The mechanism of action of CAPP treatment could be due to RONS, which trigger oxidative damage genes within the bacterial cell and ultimately lead to bacterial lysis and inactivation.