We evaluated a nonthermal plasma jet for a respective use to prevent infections from bacteria and yeasts. The plasma jet is generated from the flow of ambient air with 8 slm through a microhollow cathode discharge assembly that is operated with a direct current of 30 mA. With these parameters, the temperature in the jet reaches 43 • C at 10 mm from the discharge. Agar plates that were inoculated with Staphylococcus aureus, Pseudomonas aeruginosa, Acinetobacter baumannii, and Candida kefyr were treated at this distance, moving the plates through the jet in a meander that covered a 2 cm by 2 cm area. Different exposure times were realized by changing the speed of the movement and adjusting the distance between consecutive passes. S. aureus was most responsive to the exposure with a reduction in the number of colony forming units of 5.5 log steps in 40 s. All other microorganisms show a more gradual inactivation with exposure times. For all bacteria, a clearing of the treated area is achieved in about 2.5-3.5 min, corresponding to log-reduction factors of 5.5-6.5. Complete inactivation of the yeast requires about 7 min. Both S. aureus and C. kefyr show considerable inactivation also outside the immediate treatment area, while P. aeruginosa and A. baumannii do not. We conclude that differences in the morphologies of the membrane structures are responsible for the diverging results, together with a targeted response to different agents provided with the plasma jet. For the gram negative bacteria, we hold short-lived agents, acting across a short range, responsible, while for the other microorganisms, longer lived species seem more important. Our measurements show that neither heat, ultraviolet radiation, nor the generation of ozone can be responsible for the observed results. The most prominent long lived reaction product found is nitric oxide, which, by itself or through induced chemical reactions, might affect cell viability.

Chemical species that are generated by a plasma jet in a microhollow cathode discharge geometry when operated with air and a dc voltage were investigated. Nitric oxide (NO) is found as the dominant long-lived reaction product with concentrations of several hundreds of ppm. In comparison, the concentrations observed for nitric dioxide (NO 2 ) and ozone (O 3 ) are negligible. The concentrations of NO are increasing with increasing electric power but decreasing with increasing flow rates. Simultaneously, NO 2 concentrations are increasing slightly. The results suggest that the observed far reaching biocidal effect of the plasma jet depends on the generation of nitric oxide.

Aims:  Multidrug‐resistant opportunistic pathogens are clinically significant and require the development of new antimicrobial methods. In this study, Acinetobacter baumannii, Pseudomonas aeruginosa and Staphylococcus aureus cells were exposed to atmospheric plasma on agar plates and in vitro on porcine skin for the purpose of testing bacterial inactivation. Methods and Results:  Microbial inactivation at varying exposure durations was tested using a nonthermal plasma jet generated with a DC voltage from ambient air. The observed reduction in colony forming units was quantified as log10 reductions. Conclusions:  Direct plasma exposure significantly inactivated seeded bacterial cells by approx. 6 log10 on agar plates and 2–3 log10 on porcine skin. On agar plates, an indirect ‘bystander’ inactivation outside the plasma delivery area was also observed. The reduced inactivation observed on the skin surface was most likely due to cell protection by the variable surface architecture. Significance and Impact of Study:  Atmospheric plasma has potential for clinical application as a disinfectant of patient skin and medically relevant surfaces.

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