Antibiofilm action of a toluidine blue O-silver nanoparticle conjugate on Streptococcus mutans: a mechanism of type I photodynamic therapy

Abstract

The objective of this study was to evaluate the anti-biofilm efficacy of photodynamic therapy by conjugating a photosensitizer (TBO) with silver nanoparticles (AgNP). Streptococcus mutans was exposed to laser light (630 nm) for 70 s (9.1 J cm⁻²) in the presence of a toluidine blue O–silver nanoparticle conjugate (TBO–AgNP). The results showed a reduction in the viability of bacterial cells by 4 log₁₀. The crystal violet assay, confocal laser scanning microscopy and scanning electron microscopy revealed that the TBO–AgNP conjugates inhibited biofilm formation, increased the uptake of propidium iodide and leakage of the cellular constituents, respectively. Fluorescence spectroscopic studies confirmed the generation of OH^• as a major reactive oxygen species, indicating type I phototoxicity. Both the conjugates down-regulated the expression of biofilm related genes compared to TBO alone. Hence TBO–AgNP conjugates were found to be more phototoxic against S. mutans biofilm than TBO alone.

Keywords:

• Biofilm
• photodynamic therapy
• toluidine blue O–silver nanoparticle conjugate
• type I
• gene expression