Blue light photoinhibition of Streptococcus mutans: potential chromophores and mechanisms

The direct application of blue light (λ = 400–500 nm) provides a promising antimicrobial modality, the effects of which are mediated through generation of reactive oxygen species. Porphyrins are organic compounds essential for bacterial synthesis of heme and are understood to be the main blue light chromophores within bacteria, which are critical to the sensitivity to blue light. However, Streptococcus mutans — the principal etiological species of dental caries — has shown susceptibility towards blue light despite reportedly lacking heme synthesis pathways, raising a question as to how this susceptibility is mediated. S. mutans lacks heme-containing cytochromes for full aerobic respiration, instead relying mainly on flavin adenine dinucleotide enzymes for oxygen-dependent metabolism. This review article investigates the potential target chromophores and mechanisms underpinning the inhibitory effects of blue light in S. mutans. Multiple reports support the proposition that bacteria with blocked heme synthetic pathways still possess the genetic antecedents capable of generating porphyrins and heme proteins under appropriate conditions. Blue light is absorbed by flavins, and hence, the flavoenzymes also represent potential chromophores. In conclusion, depending on in-vitro growth and metabolic conditions, there is more than one blue light chromophore within S. mutans. To optimise clinical application of blue light-induced antimicrobial effects, future investigations should focus on in-vivo models and clinical trials.