# Low Temperature Plasma Strategies for
## 4 Discussion and Conclusions  
In the present study, we investigated the effects of Air LTP application on XfDD inactivation. Air plasma is a rather complex environment in terms of chemical composition (RONS) and light emission (from UV to mid-range IR, mainly due to molecular nitrogen systems). Using the optical emission spectrum (OES) recorded at about 3 mm from the SDBD surface, we observed weak emissions from the Nitric Oxide gamma-band covering the UV-C region (200-280 nm), from OH (mainly in the presence of water) and N2 SPS in the UV-B region (280-315 nm). Thus, direct photooxidation of the protein coat is not considered to be the dominant inactivation process. The strongest emission was observed in the UV-A region (315-400 nm), due to the SPS, FNS band systems of N2. Although we cannot exclude UV-A-mediated bacterial disinfection [82, 83], this cannot be the main process leading to cell death due to the reported exposure time and energy density. From the SPS (0,0) emission and comparison with the corresponding simulation spectra, we can also rule out heat as a possible mechanism for the disinfection of the samples. From the emission spectroscopy in the mid- IR, we also observed the presence of atomic oxygen at 777 nm. Moreover, due to the energy density used, we can also deduce that the discharge is operating in ozone mode since ozone is the main post-discharge product along with a small part of NOx present as post-discharge products. Thus, the main disinfection mechanism could be due to oxidative pressure by the reactive oxygen and nitrogen species (RONS), which are produced by plasma and are toxic to bacterial pathogens at high concentrations. These RONS oxidize proteins, lipids and nucleic acids and lead to the destruction of the pathogen or abrogate bacterial pathogenicity [84]. The efficacy of the treatment fixed at 200 s makes disinfection a rather rapid process in the case of planktonic cells. Moreover, pre-treatment of the cell culture medium with plasma does not result in sufficient changes in substrate chemistry leading to apoptosis of the cells. The presence of biofilm in the cell culture makes it more difficult to treat with only one discharge run at the selected treatment time. This can be overcome by increasing the plasma dose. Since we wanted to keep the plasma parameters constant, we decided to increase the plasma dose by repeating the treatment with a different frequency. This guarantees that we keep the temperature of the plasma phase close to room temperature and preserve the electrode from possible failure. In this case, the best results were obtained with a repeated daily treatment for 1 week, which stopped cell growth and resulted in at least a log-2 lower growth in bacterial colonies to untreated samples. For the practical use of LTP, the bacteria must be treated in their natural habitat, the xylem vessels of the plant. To meet these biological requirements, we investigated the inactivation properties of PAW in liquid culture. The nontoxic effect of PAW on plants has been tested in side experiments on tomato plants (results not shown) and also reported in recent literature making this approach a possible active medium to control XfDD in plants [85, 86]. In our experiment, we used freshly prepared PAW obtained by activation of sterile water by SBDB in air. Assuming that the activity of PAW in terms of sterilization properties is due to the presence of peroxnitroous acid (which requires nitrite, peroxide and acidic conditions) [53], freshly prepared PAW by cold plasma processes should be the most efficient means. Chemical analysis of freshly prepared PAW at a treatment time of 15 min showed the presence of nitrite, nitrate and hydrogen peroxide. The high nitrate, nitrite and peroxide content of the freshly produced PAW was indicative of the best treatment efficacy in this case.