## 3 Results and Discussion
### Bacterial Inactivation Effect and Kinetics Model Analysis
#### 3.1.2 Embedding _S. Typhimurium_ and _S. aureus_ in Liquid Media  
Survival curves of _S. typhimurium_ and _S. aureus_ in liquid media are shown in Figure 1b-d, and Figure 2b-d. The survival curves exhibited a shoulder region followed by a log-linear phase regardless of liquid medium and bacterial species. The shoulder region has been attributed to the time required by generated germicidal agents in NTP treatments to reach the minimum concentrations on cell membranes for causing damages to bacterial cells [29]. Hence, the differences in shoulder phase length or duration reflect the requirement of limiting levels of germicidal agents to start inactivating bacteria or thresholds for formed germicidal agents to start damaging microbial cells on different media. Once the concentration of germicidal agents passes the threshold, a log-linear phase is triggered, resulting in irreversible damage and lysis of the cells [29]. The kinetics parameters from Tables 1 and 2 for _S. typhimurium_ and _S. aureus_, respectively, show that compared with buffer liquid system, a shorter shoulder and a higher maximum specific inactivation rate were found with salt solutions. In addition, no visible colony formation was detected in 2% NaCl solution after samples were treated for as short as 120 s (Figures 1b and 2b) regardless of bacterial species and recovery method. However, it took 140 seconds to finish inactivation in 0.9% NaCl solution. The plasma-liquid interaction likely generated other oxidizing species such as hypochlorous acid (HOCl) [30]. With higher concentration of NaCl in liquids, the more HOCl was produced [31]. However, for the other two media, microbial populations were still countable after 140s' treatments. These differences could be due to the acidification effect of aqueous solution by DBD-NTP. In our present study, it was noted that the pH values of the salt solution descended to 3.7 (data not shown) after NTP treatments, whereas pH of buffer liquid system did not significantly change. Acidification has been hypothesized to be one of the main mechanisms for DBD-NTP treatments to reduce microbial loads in an aqueous solution. Our results are in agreement with what was reported by [PERSON] et al. [32]. [PERSON] et al. [33] studied the effect of plasma on properties of water and concluded that the NOx formation, including nitrous HNO\({}_{2}\) and nitric acid HNO\({}_{3}\), is responsible for acidification of aqueous solution. Bacteria are negatively charged colloidal particles. Their isoelectric points are disturbed in the acidic conditions, which leads to the death of microorganism [34]. Further study with NTP [35] showed that addition of nitrate ions in aqueous solution alone was not able to inactivate microorganisms,but it somehow enhanced antimicrobial effect of nitrate, nitrite, hydrogen peroxide, peroxynitrrite, and hydroxyl radicals. In addition, DBD-NTP treatments can also produce hydrogen peroxide in high relative moisture environment. Hydrogen peroxide in acidic conditions is a strong oxidizer and can have a greater impact on the bacterial cell membrane [36]. pH effect on bacterial surviving DBD-NTP treatments was also demonstrated by the inactivation kinetics analyses conducted in this study. \(D\)-values for _S. typhimurium_ and _S. aureus_ in pH 7.3 PBS were consistently higher than those in pH 6.0 PBS (Tables 1 and 2). Peroxynitrous acid enters the bacterial by passive diffusion. Once inside the cell, ONOOH can damage DNA, lipids, and proteins via direct oxidation reactions or by decomposing to OH and NO\({}_{2}\)\(\bullet\). Peroxynitrite also reacts with CO\({}_{2}\) to form a nitrosoperoxycarbonate anion (ONOOCO\({}_{2}\)\({}^{-}\)) followed by decomposing to NO\({}_{2}\)\(\bullet\) and a carbonate radical (CO\({}_{3}\)\(\bullet\)\({}^{-}\)), which also can damage biomolecules [37]. The reaction mentioned above can occur only in acidic conditions. Coinciding with this research, \(D\)-values of _S. typhimurium_ and _S. aureus_ planktonic were significantly different between pH = 7.3 PBS and pH = 6.0 PBS.