Effect of blue light plus chlorhexidine therapy on Streptococcus mutans biofilm and its regrowth in an in vitro orthodontic model

Abstract

Fixed orthodontic appliances create areas of stagnation for dental biofilms and make it difficult to clean the teeth; therefore, there is a risk of developing incipient caries lesions during the orthodontic treatment. The objective of this study is to determine if the combination of 2 different therapies, phototherapy by blue light (BL) and the antimicrobial 0.12% chlorhexidine (CHX) on enamel, orthodontic brackets, and elastics, would reduce or inhibit mature Streptococcus mutans biofilms and their regrowth on these substrates 24hours after the application of the treatment; and if this treatment would interfere with bracket adhesion to the enamel. Biofilms of S. mutans UA159 were formed for 5-days over samples composed of a bovine enamel, orthodontic bracket, and orthodontic elastic. Then, the specimens were treated with 0.89% NaCl for 1minute, BL for 12minutes (72J/cm2), 0.12% CHX for 1minute, and BL for 12minutes, followed by 0.12% CHX for 1minute (BL+CHX). Biofilm was evaluated by colonies forming units and dry weight immediately after treatments and 24hours after treatments (regrowth). The pH of the spent media was measured on the fifth and sixth days. Biofilm formation on the samples after the treatments and regrowth was visually evaluated by confocal laser scanning microscopy. Shear bond strength (SBS) between bracket and enamel was evaluated using a universal testing machine at a crosshead speed of 1mm/min. After bonding, specimens were thermocycled (500× at 5-55°C), treated, and thermocycled again. After 5days of biofilm formation, BL+CHX significantly reduced the bacterial viability on enamel compared with NaCl (P=0.004) and BL (P=0.014). For bracket and elastic, all the treatments resulted in similar bacterial viability (P ≥0.081). In the regrowth, CHX and BL+CHX significantly reduced the bacterial viability in the enamel compared with the NaCl (P≤0.015) and BL (P≤0.013). For bracket, BL+CHX significantly reduced the bacterial viability compared with NaCl (P=0.008) and BL (P=0.009). For the elastic, BL+CHX eliminated the biofilms from the substrate. CHX and BL+CHX significantly reduced the bacterial viability 24hours after treatment for all substrates (P≤0.05). The media pH significantly increased when samples were treated with CHX and BL+CHX (P≤0.001). Confocal laser scanning microscopy images visually showed an abundant quantity of red cells in the samples treated with BL+CHX. There was no difference in the SBS between the treatments (P ≥0.932). The association between BL and CHX reduced S. mutans biofilm and its regrowth on an invitro orthodontic model and did not influence the bonding strength between bracket and enamel.