Oral bacterial inactivation using a novel low-temperature atmospheric-pressure plasma device

Atmospheric-pressure plasma is a new technology for biomedical applications. Utilization of an ionized gas (plasma) to achieve disinfection is an alternative sterilization technique that has become popular recently due to its safety, cost effectiveness, and superior performance to traditional methods. The purpose of this study was to evaluate the germicidal effectiveness of a low-temperature atmospheric-pressure plasma device by treating Enterococcus faecalis for different durations.

A novel low-temperature atmospheric-pressure plasma device was developed for this study. A suspension of E. faecalis (BCRC 10789) was standardized to 10⁷ colony-forming units (CFUs)/mL, as confirmed by an optical spectrophotometer. E. faecalis was first transferred and spread on 70 sterile cover glasses measuring 18 mm². Each batch of 10 specimens was exposed to the low-temperature plasma device and treated for 1 minute, 2 minutes, 3 minutes, 5 minutes, 10 minutes, and 15 minutes; the specimen treated for 0 minute served as the control. The cover glasses containing plasma-treated bacteria were then immersed into 10 mL deionized distilled water and vibrated with an ultrasonic device to detach the residual fluid. Bacterial colonies were finally inoculated into Luria-Bertani agar plates and cultured at 37°C for 24 hours. The numbers of bacterial colonies were counted to evaluate the germicidal efficacy of the plasma device, and the results were expressed as CFUs. Meanwhile, field emission scanning electron microscopy was performed to observe the cell morphology of E. faecalis prior to and after plasma treatment.

Quantitative analysis of sterilization revealed a reduction in the number of bacterial colonies with time duration. When specimens were treated for 10 minutes, colonies of E. faecalis decreased from 10⁵ CFUs to 10² CFUs. The sterilization D-value (90% cell reduction) of experiments was 2 minutes.

The novel low-temperature atmospheric-pressure device was capable of achieving effective sterilization of E. faecalis within a 2-minute interval. Further studies are needed to validate complete inactivation, refine the laboratory-made low-temperature plasma device, and develop a new plasma-jet device, which will be superior to traditional sterilization methods and can be used in root canal environment. This novel sterilization method can also be used as a clinical reference tool.