## 3 Results and Discussion  
Comparing two inactivation processes, an important factor is whether these processes were added in the linear or logarithmic scale. This depends on the procedures themselves (in our case, the PTW and PPA treatments) and the corresponding treatment conditions. In the best case, for hygiene aspects, both treatments act completely independently of each other. In this case, the reduction factor (\(\widetilde{RF}\)) of both treatments could be added in the logarithmic scale (\(\widetilde{RF}_{comb}=\widetilde{RF}_{add}\)). In order to identify the kind of interaction, the logarithmic scale of the combined treatments (\(\widetilde{RF}_{comb}\)) was compared with the sum in the logarithmic scale of the individual treatments (\(\widetilde{RF}_{add}\)). Therefore, if the measured reduction factor for the combined procedure (\(\widetilde{RF}_{comb}\)) was smaller than the sum of the individual procedure steps (\(\widetilde{RF}_{add}=\widetilde{RF}_{PTW}+\widetilde{RF}_{PPA}\)), then the individual inactivation steps did not work wholly independently from each other. This case is called antagonistic and is marked in Table 1 in red for better allocation (\(\widetilde{RF}_{comb}<\widetilde{RF}_{add}\)). If the result was a combined reduction factor equal to the added value (\(\widetilde{RF}_{comb}=\widetilde{RF}_{add}\)) in the experiments, the two inactivation processes operated completely independently, and this was called additive and was indicated by yellow in Table 1. If additional inactivation mechanisms were gained, which means \(\widetilde{RF}_{comb}>\widetilde{RF}_{add}\), this was called synergistic and was labeled in green in Table 1.  
Table 1 displays the reduction factors for _C. albicans_ obtained in single and combined PTW and PPA treatments. In general, the single treatment with PTW and PPA resulted in an \(\widehat{\text{RF}}\) of 1.1 and 1.6 maximum, respectively. However, the \(\widehat{\text{RF}}\) for the single PPA treatment was higher than for the single PTW treatment. The reason for this is perhaps the increased ability of PPA, a gas, to penetrate the entire exposed surface. If the combined treatment of PTW/PPA was applied to the _C. albicans_ inoculated PE stripes, surprisingly, the results showed an antagonistic result in most cases (\(\sim\)51.8%), followed by additive (\(\sim\)31.4%) and synergistic (\(\sim\)16.6%) effects. Best results were gained for combinations of the PPA treatment of 50 s + 5 min with the PTW treatment of 5 s + 1 min, of the PPA treatment of 50 s + 1 to 5 min with the PTW treatment of 50 s + 1 min, and of the PPA treatment of 15 s + 3 and 5 min as well as 50 s + 1 to 5 min with the PTW treatment of 50 s+ 3 min. Maybe the PTW treatment should be elongated to a 5 min post-treatment-time (e.g., 5 s + 5 min, 15 s + 5 min and 50 s + 5 min) to achieve an increase in the reduction. These results are in line with the ones published by [44], where different Gram-positive (_Listeria monocytogenes_) and Gram-negative bacteria (_Escherichia coli_, _Pectobacterium caroovorum_), as well as _Bacillus atrophaeus_ endospores, were investigated under comparable conditions. The inactivation effects observed after the single and especially after the combined PTW/PPA treatment strongly depended on the investigated microorganism and used treatment times. However, in all cases synergistic process windows were found and could be used for further investigations and applications. There are many possible mechanisms induced by PTW/PPA treatment, which may underlie the observed results. However, the effective inactivation mechanisms and any involved signal pathways induced by indirect plasma treatment based on PPA and PTW are still unidentified. A first aspect of inactivation could be an acidification, as is known for lactic acid [45]. Previous investigations showed pH values of 1.7 (5 s pre-treatment time), 1.5 (15 s pre-treatment time) and 1.1 (50 s pre-treatment time) for PTW [46].