Efficacy of air plasma microjet for wound sterilization

The sterilization of wounds with non-thermal plasmas, which can be operated at atmospheric pressure in ambient air offers an effective way to prevent infections. Most systems presented so far are operated with noble gases, such as argon or helium. The antimicrobial efficacy, however, is believed to be due to reactive oxygen and reactive nitrogen species that are generated in the interaction of the plasma with ambient air. Accordingly, plasmas generated in air could offer better decontamination results and faster treatment times. We have recently presented a non-thermal atmospheric pressure afterglow plasma jet, which is generated from ambient air by applying a dc high voltage of 2 kV in a microhollow cathode discharge geometry. By adjusting the gas flow rate of the jet, the expelled afterglow approaches room temperature within a few millimeters. We have investigated the effect of this plasma jet against the clinical relevant pathogens Staphylococcus aureus and Acinobacter baumannii, which are known for their antibiotic resistant strains. We also tested against Pseudomonas aeruginosa - another gram negative bacterium - and the yeast Candida kefyr. To mimic treatment conditions, the organisms were plated in petri dishes and a 2 cm × 2 cm square was exposed to the afterglow by moving the jet in a meander pattern across the area. Exposure parameters investigated were exposure time, distance from microbial/yeast-culture and humidity of the operating gas. For S. aureus we achieved complete pathogen removal, i.e. suggesting a reduction in the number of colony forming units by 6 orders of magnitude, with an exposure time of 111 s and a distance of 10 mm. A. baumannii had to be exposed for 165 s for the same treatment result. Most difficult to decontaminate proved C. kefyr, which required an exposure of 215 s for a 4-log reduction. To understand the mechanisms of the method, we studied the afterglow air plasma chemistry and found nitric oxide (NO) prevalent in high concentrations, while the generation of nitric dioxide (NO
2
) and ozone (O
3
) is negligible. Our investigations further show that chemical reactions in the liquid environment of the pathogens, such as the generation of hydroxyl ions (OH) from atomic oxygen and of hydrogen peroxide (H
2
O
2
) are crucial.