## 2 Materials and Methods
### Antimicrobial Testing  
In order to test the antibacterial properties of N-TiO\({}_{2}\), _Salmonella enterica_ subsp. _ enterica_ serovar Typhimurium strains from three different origins (FMCC B-137, human isolate epidemic; FMCC B-193, isolated from calf bowel--kindly provided by Prof. [PERSON]; FMCC B-194, mutant derived from B-193--kindly provided by Prof. [PERSON]) were used. The microorganisms were previously stored at \(-\)80 \({}^{\circ}\)C and were then resuscitated by adding brain heart infusion (BHI) broth (BH broth; Merck, Darmstadt, Germany). These pre-cultures were incubated for 24 h at 37 \({}^{\circ}\)C, and then an amount of them was added to BHI broth and incubated for another 18 h. These final cultures were centrifuged (5000 rpm, 10 min, 4 \({}^{\circ}\)C), washed and, finally, all of the strains of different origins were suspended in [PERSON]'s solution (Ringer's tablets; Merck, Darmstadt, Germany). Stainless steel coupons (3 \(\times\) 1 \(\times\) 0.1 cm) were soaked in acetone and autoclaved at 121 \({}^{\circ}\)C for 15 min so that they were cleaned and sterilized. To achieve the formation of the biofilm on these coupons, [PERSON]'s solution containing the inocula of previously prepared strains was incubated with the ss coupons for 3 h at 15 \({}^{\circ}\)C, in order for the bacteria to attach to the ss surface [26]. The coupons were removed from the bacterial suspension with sterile forceps and were transferred to a half-tube containing [PERSON]'s solution [26]. The test tube rack was shaken, so that the loosely attached bacteria were removed, and the coupons were transferred intoa half-tube with tryptone soy broth (TSB; LABM; International Diagnostics Group Plc, Bury, Lancashire, UK). Then, they were incubated for 48 h at 15 \({}^{\circ}\)C [26].  
After the biofilm formation, the coupons were placed into a batch, rectangular, and horizontal photocatalytic reactor (dimensions 0.5 \(\times\) 0.4 \(\times\) 0.3 m), equipped on top with a white LED light tape of 6 W/m, 870 lm/m, and 12 V, with and without N-TiO\({}_{2}\) NPs. The photoreceptor was built with reflective walls (I\({}_{\text{Cutoff}}\) = 320 nm), and all of the experiments were conducted under visible-light irradiation. After 1, 2, and 3 h of irradiation, the coupons were removed from the reactor with sterile forceps, so that they were removed aseptically, and then they were rinsed by pipetting twice with an 10 mL of quarter-strength Ringer's solution. Afterwards, the coupons were immersed for 5 min in a sterile Falcon tube containing 5 mL of Ringer's solution and 10 sterile glass beads, and they were shaken so that the attached cells were removed via bead-vortexing for 2 min. These detached cells were then enumerated by the agar plating method, after which they were serially diluted. An appropriate amount of the bacterial Ringer's solution was distributed on tryptone soy agar (TSA, LAB M, Bury, Lancashire, UK) plates and then incubated at 37 \({}^{\circ}\)C for 24 h to assess the remaining biofilm cells, with the help of a cell counting device [26].  
Coupons with _Salmonella_ biofilms without the presence of N-TiO\({}_{2}\), under visible-light irradiation and in the dark, as well as coupons with N-TiO\({}_{2}\) NPs in the dark for 0, 1, 2, and 3 h of irradiation, were used as controls, for a more holistic approach to the study.  
Every set of experiments was repeated twice (two independent experiments) and included three replicate samples (coupons) to ensure repeatability (a total of six different replicate samples).