Effect of methylene blue-induced photodynamic therapy on a Streptococcus mutans biofilm model

Several studies have reported the use of antimicrobial photodynamic therapy (aPDT) to control biofilm but its efficacy depends on several factors, such as biofilm model used. This study aims to examine whether exposure to diode laser combined with methylene blue affects the bacterial viability and polysaccharide content in a Streptococcus mutans cariogenic biofilm model, which simulated 'feast-famine' episodes of exposure to sucrose that occur in the oral cavity.

S. mutans biofilms were formed on acrylic resin discs and exposed to a 10% sucrose solution for 1min, eight times/day. After growing for 48h, the biofilms were submitted to the following treatments, twice daily (n=4): (i) 0.9% NaCl (NaCl) as the negative control; (ii) 0.12% chlorhexidine digluconate (CHX) as the positive antibacterial control; (iii) diode laser combined with methylene blue, using an energy density of 320J/cm² (aPDT). After 120h of growth, the biofilm formed on each disc was collected to determine the viable bacterial counts and concentration of insoluble exopolysaccharides (IEPS) and intracellular polysaccharides (IPS).

Bacterial counts in the biofilms formed differed among the treatments. Compared with NaCl, aPDT significantly destabilized biofilm (p<0.0001). aPDT and CHX equally lowered the concentration of IEPS and IPS in biofilms.

Under the experimental conditions assessed, our findings indicate that a twice-daily treatment with diode laser combined with methylene blue effectively decreased bacterial viability and the intra- and extracellular polysaccharide concentration in biofilms of S. mutans, a cariogenic bacterium.