*2.7. Antibacterial Evaluation*

*E. coli* and *S. aureus* were chosen to explore the antibacterial properties of gelatin/ZnO fibers by using the viable colony count method. Tryptic Soy Broth (TSB) and Trypticase Soy Agar (TSA) were used as culturing nutrient sources. Specifically, *E. coli* and *S. aureus* were aseptically inoculated in TSB and then cultured in a shaker at 37 ◦C for 16 h. Each of the cultured broths were continually cultured in new TBS in a shaker at 37 ◦C for another 3 h, and then each of the cultured broths were centrifuged and washed twice with sterile PBS (0.01 M, pH = 7.2–7.4). Finally, the bacterial PBS suspension with the concentration of 1 × 106 colony forming units per milliliter (CFU·mL−1) was obtained by the gradient dispersion method. The sterile gelatin/ZnO fibers (GZ0, GZ1 and GZ2) were cut into the shape of 2 × 2 cm<sup>2</sup> (30 mg) and then placed into tube containing 10 mL sterile PBS. One group of tubes was irradiated for 1 h under ultraviolet light (UV, 365 nm, 50 w), the other group was not irradiated. After the pretreatment, 0.1 mL of each bacterial suspension was added, and incubated in the shaker at 37 ◦C for 3 h. After that, a 10 μL solution was taken and serially diluted in sterile PBS. Then, 30 μL of each diluent was taken and spread onto a TSA plate, and then all plates were incubated for 16 h at 37 ◦C. The numbers of the suitable colonies that formed were counted. In addition, medium with only inoculum was used as negative control, and pure ZnO particles (30 mg) were used as positive control. All experiments were performed in triplicate.
