Antibacterial cold gas plasma at safe levels for skin cells in vitro

Bacterial contamination is a constant threat in burn wound care and needs to be treated. It is however important to keep a balance between inactivating the bacteria and maintaining the wound healing potential. Cold atmospheric pressure plasmas might provide additional means to reduce the bacterial load in a burn wound. Cold plasmas contain various reactive components, which together are effective in killing bacteria. We studied the effects and mechanism of cold argon plasma treatment on in vitro inactivation of Pseudomonas aeruginosa, which is commonly isolated from burn wounds. Bacteria were treated in physiological salt with a pulsed argon plasma. Surviving bacteria were counted by plating dilutions. Safety was assessed by treating cell cultures of keratinocytes or fibroblasts similar to the bacteria. Viability was estimated by measuring cell activity with a tetrazolium based assay. Gas temperature, absolute UV irradiance, emitted ions and induced liquid chemistry were measured. Bacterial inactivation reached up to 6-log reduction after 2 min. Effective plasma treatment was highly dependent on acidification of the samples and was further related to the distance, duty cycle and treatment time. Heat, metastables, ion- and UV-fluxes, electric fields, and gas flow did not to have a prominent direct effect on bacterial inactivation. Rather, plasma exerts its effect through liquid phase chemistry (HNO2, ONOO- and H2O2). Activity of fibroblasts or keratinocytes was not reduced by plasma treatment using the same conditions. Extended incubation for 2 hours in plasma treated solution did however reduce cell activity. Preliminary experiments with plasma treatment of burn wounds on ex vivo human skin showed a trend towards improved reepithelialization. Nonthermal argon plasma can be used to rapidly kill bacteria and yet preserve the viability of skin cells. The bactericidal effect can be solely ascribed to plasma induced liquid chemistry, leading to the production of reactive chemical species.