A study of plasma treatments effects on dental biofilms

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Dental plaque is a biofilm that grows on surfaces within the mouth and contains millions of bacteria. The bacteria in plaque cause tooth decay and gum disease. In order to inactivate dental plaque, alternative and more efficient disinfection strategies are demanded. Non-thermal atmospheric plasma gains rising attention with its promising antimicrobial properties. To date, however the effectiveness and mechanism of plasma disinfection of biofilms are not fully understood. This study focuses on investigation of the treatment effects of non-thermal atmospheric plasmas on dental biofilm disinfection. Various dental biofilm models, including Streptococcus mutans (S. mutans) biofilm, Candida albicans (C. albicans) biofilm and Porphyromonas gingivalis (P. gigivalis) biofilm, were first developed in order to study the plasma treatment effects. The plasma treatment showed effectiveness of disinfection against gam-positive bacterial (S. mutans) biofilms, gram-negative bacterial (P. gingivalis) biofilms and fungal (C. albicans) biofilms. MTT assay revealed that there were 90% S. mutans biofilm reduction, 80% P. gingivalis biofilm reduction and 80% C. albicans biofilm reduction due to the plasma treatment. The disinfection efficacy was depending on the plasma exposure time. Confocal laser scanning microscope (CLSM) examination indicated that the plasma treatment could penetrated into the biofilms. Scanning electron microscope (SEM) examination showed that plasma treatment induced severe destruction of bacteria cells. Research effort was also made to study the bacterial recovery performance by recovering the biofilms after the plasma treatment. It was found that plasma treatment could not only inhibit biofilm recovery, but also change the physiological status of the bacteria in the biofilms. It was found that the recovered biofilm after plasma treatment displayed higher metabolic activities and was more susceptible to antibiotics and oxidative stresses. The aconitase activity in plasma group increased by 65% as compared with the untreated control group. When plasma treated S. mutans biofilms were recovering with ciprofloxacin, there is 3.6 log reduction by CFU counting as compared to that of the regular biofilms. Besides the direct plasma treatment on biofilms, the plasma brush was also used to activate various liquid, which would produce antimicrobial agents. The plasma activated liquids also demonstrated great disinfection effect against the dental biofilms. 40% bacteria reduction was detected when the biofilm was treated by plasma activated liquid. Moreover, plasma activated liquid did not change cell viability base on the MTT assay results. It implied the plasma activated liquid is not toxic for mammalian cells. In conclusion, non-thermal atmospheric plasma treatment is very effective in both direct disinfection of dental biofilms and their recovery performance after the treatment. The experimental results obtained in this study further demonstrated that non-thermal atmospheric plasmas have great potential in dental clinic applications.