2.2. The Filter Paper Disk Agar Diffusion Method

The antibacterial activity of GERM CLEAN (Shanxin, Chengdu, Sichuan, China) on S. mutans was preliminarily tested using the filter paper disc agar diffusion method introduced elsewhere with minor modification [44–47]. Briefly, 100 μl of mid-exponential phase UA159 suspension was spread on fresh nutrient BHI agar medium plates and dried at room temperature for 5 min. The 5 mm sterile filter paper disks were impregnated with GERM CLEAN (the stock solution) for 5 s and then were applied to the surface of above BHI bacterial culture plates. Plates were then incubated for 24 h at 37°C. The antibacterial activity was evaluated by measuring the diameter of the inhibition zone.

The experiments were repeated three times independently.