## 4 Discussion
Conversely, the yeasts' repartition was heterogeneous on resin samples, with many thick clusters and some non-contaminated areas (Figure 3). The resin hydrophobicity and surface roughness could explain this specific adhesion of _C. albicans_[6; 7; 60]. Indeed, destroying those clusters was more difficult due to their thickness and the presence of an extracellular matrix. Hence, the deeper yeast layers were better protected from NTP [37; 40; 61]. According to many authors [38; 40; 53], increasing the treatment time or producing intracellular ROS in biofilm [62] would favor the destructuration of those clusters. Despite the clusters on resin surface, our NTP plasma led to a significant 6-log viability reduction after a 120-min treatment in low working pressure. This efficacy could be explained by the generation of vacuum ultraviolet (VUV) photons in low-pressure plasma conditions only [54; 63]. These VUV photons are highly energetic due to their short wavelength (from 100 to 200 nm). Thus, VUV photons and previously described species may act together and have a considerable impact on biofilms. However, a better comprehension of the mechanism of active plasma species could help to optimize the treatment. Concerning the resin color stability, chemical cleansers used to decontaminate dentures lead to adverse effects when repeated or prolonged treatments are applied [18; 19; 21; 24; 25; 27; 29; 31; 32]. The most frequent resin color changes are bleaching using sodium hypochlorite [16; 19; 21; 27; 29] and alkaline peroxides [19; 24; 25; 27; 30; 31]. In the same way, one in vitro study highlighted resin staining after prolonged treatment using lemongrass extract [18]. The evaluation of color change is related to ISO technical report (TR) 28642:2016, which defined two color distance metrics \(\Delta\)E\({}^{*}_{\text{ab}}\) and \(\Delta\)E\({}_{00}\). It only defines a perceptibility threshold (PT) of \(\Delta\)E\({}^{*}_{\text{ab}}\) = 1.2 and an acceptability threshold (AT) of \(\Delta\)E\({}^{*}_{\text{ab}}\) = 2.7. This technical report does not concern the color change of resins for denture base, but only dental ceramics, dental resins or silicone elastomers. The previous authors mainly used \(\Delta\)E\({}^{*}_{\text{ab}}\) to evaluate the effect of the different cleansers on resin color. However, those studies presented many discrepancies with respect to devices and methodologies, which led to disagreements upon PT and AT values [48]. Hence, [PERSON] et al. [47; 48] carried out a study using \(\Delta\)E\({}_{00}\) to specifically determine the PT and the AT of color changes, after accelerated aging of an acrylic denture base resin: PT value was 1.71\(\Delta\)E\({}_{00}\) and AT value was 4.00\(\Delta\)E\({}_{00}\). Like [PERSON] et al., we investigated the possible adverse effects of our O\({}_{2}\) NTP process on the color of ProBase(tm)Hot 36 P-V resin (Ivocalr Vivadent), using \(\Delta\)E\({}_{00}\). The spectrophotometry and the visual analyses showed a slight variation of the resin color within each sample in all groups. This could involve the presence of red fibers in the resin that mimic blood vessels within the mucosa, improving the denture aesthetics. As a result, biases may exist during spectrophotometric measurements when repositioning the sample before and after treatments. The spectrophotometry analysis showed no significant color change (\(\Delta\)E) of the resin when exposed to 120-min low-pressure or one O\({}_{2}\) NTP treatment. However, a significant increase of \(\Delta\)E occurred after six repeated treatments (1.33, \(p\) < 0.05). This significant color change involved a significant increase of the lightness (L*) and a higher proportion of red and yellow color components (a* and b*), which slightly increased the color chroma. However, \(\Delta\)E value was under the acceptability threshold for acrylic denture base resins (<4.00) recently determined by [PERSON] et al. [47; 48]. Moreover, this color modification was not perceptible by any of the three qualified observers.