Effect of Phototherapy on Streptococcus mutans Metabolism and Protein Expression

Objectives: The aim of this in vitro study was to determine the effect of violet-blue light on metabolic activity of early Streptococcus mutans biofilm, reincubated at 0, 2 and 6 h after 5 min of light treatment. A secondary objective was to determine glucan binding protein B (GbpB) expression in response to violet-blue light treatment.
Methods: S. mutans UA159 biofilm grew for 12-16 h in microtiter plates in Tryptic Soy broth (TSB) or TSB with 1% sucrose (TSBS). Biofilm was irradiated for 5 min with violet-blue light at 405 nm. After irradiation, microplates were reincubated at 37°C for 0, 2 or 6 h in 5% CO₂. A colorimetric tetrazolium salt (XTT) assay with menadione was used to investigate bacterial metabolic activity. Total protein was extracted, quantified and separated by electrophoresis. Separated proteins were transferred to PVDF membranes, blocked with 5% non-fat milk, and probed for GbpB with primary rat monoclonal and anti-rat IgG secondary antibodies. A mixed model ANOVA was used to test the effect of light at 0, 2 and 6 h in TSB and TSBS. Student t test was used to determine the expression of GbpB in TSB or TSBS.
Results: Bacterial metabolic activity was significantly lower in the violet-blue light group than the non-treated (p<0.0001) group regardless of media or time. For TSB, 0h had significantly lower metabolic activity than 2 (p=0.0049) and 6 h (p=0.0144) but 2 and 6 h were not significantly different from each other (p=0.58). GbpB expression was significantly lower (p = 0.018) in the TSBS group but not in TSB.
Conclusions: Violet-blue light inhibits S. mutans biofilm and affects GbpB expression in sucrose and no sucrose conditions.