Zinc enhances the phototoxic effect of blue light against malodour-producing bacteria in an experimental oral biofilm

Oral malodour is thought to be caused mainly by the production of volatile sulfide compounds (VSCs) by anaerobic Gram-negative oral bacteria. Previous studies have shown that these bacteria are susceptible to blue light (400–500 nm wavelength). In the present study, we tested the effect of blue light in the presence of zinc, erythrosine B or both on malodour production in an experimental oral biofilm. Biofilms were exposed to a plasma-arc light source for 30, 60 and 120 s (equal to energy fluxes of 41, 82 and 164 J cm⁻², respectively) with or without the addition of zinc acetate, erythrosine B or both. After the light exposure, biofilm samples were examined for malodour production (by an odour judge) and VSC production (with a Halimeter), and VSC-producing bacteria were quantified using a microscopy-based sulfide assay (MSA) and in situ confocal laser scanning microscopy (CLSM). Results showed that exposing experimental oral biofilm to both blue light and zinc reduced malodour production, which coincided with a reduction in VSC-producing bacteria in the biofilm. These results suggest that zinc enhances the phototoxicity of blue light against malodour-producing bacteria.