## 3 Results
### Effect of the Generated (Reactive) CAP Species on the Biofilms  
#### 3.2.1 Viable Cell Density Reduction Due to UV Light  
As mentioned before, biofilm samples were covered with a quartz disc to only allow the UV photons to interact with the model biofilms. Log-reductions were calculated based on the remaining viable population following CAP treatment and the cell density of the untreated model biofilms. These initial cell densities were 7.10 \(\pm\) 0.32 and 6.10 \(\pm\) 0.34 log\({}_{10}\)(CFU/cm\({}^{2}\)) for the untreated _Listeria monocytogenes_  
Figure 5: Fluorescence measurements (-) obtained following CAP and DCFH2-DA treatment of the _Listeria monocytogenes_ and _Salmonella_ Typhimuriumium model biofilms (_n_ = 3). For each species, significant differences between all examined conditions (ANOVA test 1), including the untreated control, have been indicated by means of a different small letter, with ‘a’ bearing the lowest value. The influence of the oxygen level (ANOVA tests 2–5), the electrode configuration (ANOVA tests 6–9), and the discharge power (ANOVA tests 10–13) have been indicated as well by means of a different small letter, with ‘a’ again bearing the lowest value. Separate ANOVA tests were performed for each biofilm forming species. and _Salmonella_ Typhimurium model biofilms, respectively. These log-reductions (with implementation of the quartz disc) were compared with those previously observed without implementation of the quartz disc [17] and this comparison can be observed in Figure 6 for both model biofilms. Separate ANOVA tests were performed to (i) compare the log-reductions obtained with and without quartz disc while applying the same plasma characteristics, and (ii) to examine the influence of one plasma characteristics on the log-reductions obtained with implementation of the quartz disc while keeping the other influencing factors constant. Based on Figure 6, it can be concluded that the log-reductions obtained with the implementation of the quartz disc were quite low, i.e., the values were generally lower than 1 log\({}_{10}\)(CFU/cm\({}^{2}\)). Comparing the log-reduction results obtained with and without the implementation of the quartz disc, it can be observed that only two CAP treatment conditions resulted in significant differences for the _Listeria monocytogenes_ model biofilm, i.e., significantly higher log-reduction values were obtained without the quartz disc for SBD-0.0% (_v/v_)-21.88 V and SBD-1.0% (_v/v_)-21.88 V. For the _Salmonella_ Typhimurium model biofilms, on the other hand, log-reduction values without quartz disc were generally higher than the corresponding values with quartz disc. Only three exceptions were observed, i.e., no significant differences were observed for DBD-1.0% (_v/v_)-13.88 V, SBD-0.0% (_v/v_)-13.88 V, and SBD-1.0% (_v/v_)-13.88 V.  
Regarding the influence of the plasma characteristics on the contribution of UV light to the CAP inactivation efficacy, it can be concluded that the oxygen level generally did not influence the log-reduction values obtained with the implementation of the quartz disc. Only one exception can be observed for the _Salmonella_ Typhimurium model biofilms, i.e., while using the SBD and the highest discharge power, higher values were obtained without the addition of oxygen. The influence of the electrode configuration was rather limited as well. Significantly higher values using the DBD electrode were only observed for two conditions, i.e., (i) for _Listeria monocytogenes_ using a helium gas flow and  
Figure 6: Log-reductions (log\({}_{10}\)(CFU/cm\({}^{2}\))) obtained following CAP treatment of the model biofilms using each possible combination of plasma characteristics (_n_ = 2). Log-reductions with quartz disc were compared with those previously obtained without quartz disc [17] to comment on the contribution of UV light to the inactivation efficacy of the CAP treatment.