Exposure to diode laser (810nm) affects the bacterial adherence and biofilm formation in a E. faecalis biofilm model

A large number of studies have shown that diode laser can effectively kill Enterococcus faecalis (E. faecalis). However, to our knowledge there has been little information regarding high-level analysis of sterilization mechanism on E. faecalis biofilm models after laser irradiation. This study provides emphasis on the E. faecalis viability and exopolysaccharide content after laser irradiation. It also aims to examine whether diode laser affects the biofilm formation and adherence of E. faecalis biofilm at the level of gene expression.

E. faecalis (ATCC 29,212) was inoculated for biofilm formation. After growing for 48 h, the biofilms were submitted to the following treatments, twice daily (n = 6): (i) Diode laser with energy densities as 28.7 J/cm²; (ii) 1% sodium hypochlorite (NaOCl) was used as a positive control; (iii) 0.9 % physiological saline (NaCl) was used as a negative control. After 120 h of biofilm growth, bactericidal activity of diode laser was evaluated by using plate count method and scanning electron microscopy (SEM). The distribution of extracellular polysaccharide was assessed by Confocal laser scanning microscopy (CLSM). Anthrone method was used to quantify the amount of water-soluble exopolysaccharide (WSE) and water-insoluble exopolysaccharides (WIE) in E. faecalis. Real-time quantitative polymerase chain reaction (RT-qPCR) was employed to assess transcription of genes (gelE, ace and esp) related to formation and adherence of E. faecalis biofilm.

Compared with NaCl, diode laser significantly destabilizedE. faecalis biofilm, which showing a decrease in the number of bacteria and inhibiting the adherence of biofilm (p < 0.05). From Anthrone method and CLSM, the study found that diode laser lowered the concentration of WSE and WIE in biofilms (p < 0.05). In addition, transcription of gene gelE, ace and esp were also reduced after irradiation of diode laser (p < 0.05).

By inhibiting the synthesis of WIE and WSE, diode laser may reduce the formation of E. faecalis biofilm. The expression of specific genes (gelE, ace and esp) involved in bacterial adherence and biofilm formation were down regulated under the exposure to diode laser (810 nm), which in turn is expected to decrease the pathogenicity of E. faecalis.