## 3 Results and Discussion
### Inactivation of Salmonella and Listeria monocytogenes
A primary target for the cold plasma reactive species is the cell membrane of bacteria, resulting in oxidative damage, affecting its permeability and loss of membrane potential [30; 31], denaturing functional proteins [32], damaging DNA [11; 33; 34], and causing accumulative injury in a time-dependent manner surpassing the anti-oxidative stress capacity of the bacteria [35]. The food-processing facility's conditions can affect the effectiveness of cell inactivation by CAP. In this study, BSA was used to simulate soiling on inoculated SS surfaces. According to our results, CAP treatment of protein-soiled samples achieved a lower reduction compared to clean SS. After 300 s of CAP treatment, a difference of 0.80-0.85 log CFU/mL for \(L\). _monocytogenes_ and 0.84-1.22 log CFU/mL for _Salmonella_ was observed between clean and protein-soiled samples. Similarly, [PERSON] et al. observed that inactivation levels were higher using a high-voltage CAP in clean conditions compared to a protein-soiled surface achieving a log CFU difference of 1.72 log CFU/mL for \(L\). _monocytogenes_ after 175 s [3]. It has also been found that pathogenic bacteria covered with BSA and subsequently inoculated on stainless steel also showed reduced inactivation levels after cold plasma treatment. This was attributed to the protective effect of serum albumin on the bacterial cells against the reactive species generated by CAP [29]. The current results show that a shorter distance from the plasma source, i.e., 10 mm, resulted in higher inactivation levels compared to a 20 mm distance. For example, this can be shown by the reduction achieved for \(L\). _monocytogenes_ after 60 s plasma exposure at 10 mm (0.81-1.51 log CFU/mL) and 20 mm (0.48-1.01 log CFU/mL). A similar trend was observed for _Salmonella_ as well. However, it is noteworthy that the difference in the achieved pathogen reductions between the two different treatment distances decreased with longer CAP exposure times. This phenomenon could be attributed to the higher accumulation of reactive species due to longer CAP exposure. Spatiotemporal studies have shown that plasma reactive species (e.g., NO\(\bullet\), OH\(\bullet\)) can be transported several millimeters beyond the plasma source during treatment [36; 37]. However, the distance from the plasma source can affect the antimicrobial effectiveness due to changes in the mass transport of the different reactive species from the discharge layer to the target surface. Comparably to this study, [PERSON] et al. and [PERSON] et al. reported a negative correlation between the distance from the plasma source and the inactivation rate on SS inoculation [3; 38]. However, [PERSON] et al. achieved an optimum inactivation at 20 mm of spacing compared to 10 and 30 mm using a pressure plasma jet on raw chicken breast and a mixture of N\({}_{2}\) + O\({}_{2}\) as working gas [39]. These differences could be due to differences in the topography of the surface and the plasma source. In general, a shorter distance would enhance bacterial inactivation by increasing the concentration of the reactive species and their contact with the cell wall. Although, in general, \(L\). _monocytogenes_ appeared to be more resistant to CAP treatment, no significant differences in reduction levels (i.e., \(p\) \(>\) 0.1) were found between the two pathogens used in this study. These findings differ from the results observed by [PERSON] et al., 2018, where Gram-positive strains such as _Klebsiella pneumoniae_ have shown more resistance to CAP treatment compared to the Gram-negative _Y. enterocolitica_[29]. This difference was attributed to the thickness of the outer cell wall, which is thicker in Gram-positive bacteria. However, other factors such as the shape of bacteria, where rods are more sensitive than cocci [40], and the type of plasma source [22], could introduce differences in the susceptibility of bacteria.