## 3 Results and Discussion
### Antimicrobial Assessment  
The results of the antimicrobial testing for the N-TiO\({}_{2}\) NPs under visible-light irradiation are presented in Figure 7.  
An effect of the presence of N-TiO\({}_{2}\) on the _Salmonella_ Typhimurium strains was observed. In detail, under visible-light irradiation, the N-TiO\({}_{2}\) NPs led to a one-logarithm decrease in the biofilm population. The inhibition of proliferation was more obvious after 1 and 2 h of irradiation, whereas after 3 h the populations gradually started to increase again. This was the case for all of the biofilm specimens, as _Salmonella_ Typhimurium shows great resistance as a bacterium. When compared to the biofilm in a dispersion of N-TiO\({}_{2}\) under no irradiation, the NPs' photoactivity was always evident, for all three different times of irradiation.  
It is noteworthy that, in some cases, there was a tendency for the N-TiO\({}_{2}\) NPs to attract greater biofilm populations. This occurred both under visible-light radiation and under no irradiation. For example, in the first hour of irradiation, the biofilm population was slightly higher when N-TiO\({}_{2}\) was present, compared with the control specimen in the photoreceptor. Similar results were obtained for the biofilms left in the dark for 1 h. The reason for this was that the NPs, due to their morphology and small size (i.e., higher surface area), offered a wider space for the biofilms to grow on. Moreover, they created \"trapping\" sites that worked as shelters for the cells.  
The abovementioned results contrast somewhat with the recently published literature [17; 24; 44], since all of the published studies included testing in free-standing bacterial strains. This is a preliminary attempt to test the antimicrobial properties of N-TiO\({}_{2}\) NPs under visible-light irradiation in biofilms. The need to further investigate N-TiO\({}_{2}\) NPs is of high importance in the biomedical and dental fields [45; 46], and previous research from our study group has already investigated the effects of nanoparticles on the formation and surface modification of _Salmonella_ biofilms [47].  
Consequently, the morphology of the N-TiO\({}_{2}\) photocatalyst needed to be further investigated so that the biofilm testing protocol could be optimized. This was due to the existence of antimicrobial properties of N-TiO\({}_{2}\)--even in that form--when the visible light source was present, compared to when it was missing. The percentage of remaining _Salmonella_ Typhimurium biofilm cells for every irradiation time is shown in Figure 8.  
The results presented in Figure 8 were derived from comparing the viable cells on the biofilms with N-TiO\({}_{2}\) NPs irradiated with visible light to the biofilms with N-TiO\({}_{2}\) NPs in dark. After 1 h of visible-light irradiation, the percentage of viable (remaining) biofilm cells was 6%, which means that there was a 94% killing of _Salmonella_ Typhimurium biofilm cells  
Figure 8: Percentage of remaining viable _Salmonella_ Typhimurium biofilm cells after 1, 2, and 3 h of visible-light irradiation, with the presence of N-TiO\({}_{2}\) NPs.  
by the N-TiO\({}_{2}\) NPs. As the _Salmonella_ Typhimurium biofilm manifested its resistance to cell death, the percentage of remaining cells after two hours of irradiation decreased to 61%, or 39% killing. After three hours of irradiation, 42% of cells remained, i.e., 58% killing of biofilm cells.