## 3 Results and Discussion
### Antimicrobial Properties of the Plasma-Treated Liquids  
In order to investigate the antimicrobial effect, a benchmark test was performed with plasma-treated physiological saline solution and the gram-negative bacterium _E. coli_ K-12 DSM 11250/NCTC 10538. During the experimental procedure, the liquid was treated with plasma for 10, 20, or 30 min. The microorganisms were exposed to the plasma-treated liquid for up to 30 or 60 min (Figure 8; data for 60 min not shown). The exposure of the microorganism _E. coli_ to physiological saline solution treated with plasma for 10, 20 or 30 min resulted in a reduction of 5.3 orders of magnitude (log\({}_{10}\) cfu/mL) within t\({}_{\rm exp}\) = 30-60 min. The inactivation of _E. coli_ was dependent on the treatment time. Thus, within an exposure time of 10 min a reduction of total viable cell count of 0.5 orders of magnitude was determined for t\({}_{\rm treat}\) = 10 min whereas for t\({}_{\rm treat}\) = 30 min already a reduction of 2.3 orders of magnitude was reached. The fastest inactivation for _E. coli_ occurred with the 30 min plasma-treated saline solution. For this  
Figure 8: Antimicrobial effect of plasma-treated saline solution on the gram-negative bacterium _E. coli_ for different treatment times: t\({}_{\rm treat}\) = 10, 20, and 30 min, determined by total viable count [cfu/mL] within an exposure time t\({}_{\rm exp}\) = 30 min. Figure 7: Change of conductivity with treatment time. treatment time, no microorganisms were detected after \(\mathrm{t_{exp}}\) = 30 min and for \(\mathrm{t_{treat}}\) = 20 min, the residual viable count was negligible (3.9 cfu/mL) after the same exposure time (Figure 8). Moreover, a prolongation of \(\mathrm{t_{exp}}\) to 60 min resulted also for \(\mathrm{t_{treat}}\) = 10 min in complete inactivation of tested microorganisms (data not shown). Thus, the antimicrobial effect of the plasma-treated liquid was dependent on the treatment time and the exposure time. The inactivation of the microorganisms was faster with longer treatment time or exposure time. These two factors can be used for the regulation of the antimicrobial activity. This correlation of treatment time and antimicrobial activity has been shown previously for other plasma sources [18; 36; 37]. The results of the benchmark test showed that the antimicrobial effect of the plasma-treated saline solution was strong enough to inactivate successfully _E. coli_ in all experiments performed. The antimicrobial properties of plasma-treated water like deionized or sterilized water have already been shown by [PERSON] et al. [9] and [PERSON] et al. [17]. Thus, we investigated the antimicrobial effect of tap water, which differed in the important fact that it shows no decrease of the pH-value due to plasma treatment (Figure 6). Again, the microorganism _E. coli_ was used. Additional experiments were performed with the gram-positive bacterium _S. aureus_. The results of the experiments with tap water were compared with those of plasma-treated saline solution. Since the liquid treated for 30 min showed the strongest antimicrobial effect we decided to use \(\mathrm{t_{treat}}\) = 30 min in all experiments. The reductions of the total viable count of _E. coli_ and _S. aureus_ in plasma-treated tap water were more than 5 orders of magnitude (Figure 9). In general, it was found that the level as well the kinetics of inactivation of the microorganisms was dependent on the kind of plasma-treated liquid. Thus, the inactivation by 5 orders of magnitude was achieved for _E. coli_ in saline solution after \(\mathrm{t_{exp}}\) = 15 min whereas for tap water already \(\mathrm{t_{exp}}\) = 5 min resulted in the same reduction. In the case of _S. aureus_, after \(\mathrm{t_{exp}}\) = 30 min, the reduction was approximately 3 orders of magnitude in plasma-treated saline solution but again 5 orders of magnitude for plasma-treated tap water. The plasma-treated tap water was much more effective against the tested microorganisms _E. coli_ and _S. aureus_ than saline solution.