Blue light absorbing pigment in Streptococcus agalactiae does not potentiate the antimicrobial effect of pulsed 450 nm light

Introduction: Recently, it was shown that Group B Streptococcus (GBS) COH1 strain, which has granadaene—an endogenous chromophore known to absorb blue light—is not susceptible to 450 nm pulsed blue light (PBL) inactivation unless the bacterium is co-cultured with exogenous porphyrin. Purpose: To confirm or refute the finding, we studied the effect of blue light on NCTC, another strain of GBS with more granadaene than COH1, to determine if the abundance of granadaene—and by implication more absorption of blue light—fosters GBS susceptibility to PBL. Methods: We irradiated cultures of the bacterium with or without protoporphyrin, coproporphyrin, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD) or NADH. After 24-h incubation, bacterial colonies were enumerated, log₁₀ CFU/mL computed, and descriptive and inferential data analyzed and compared. Results: (1) The rich amount of granadaene in NCTC did not enhance its susceptibility to antimicrobial pulsed blue light (PBL). (2) Adding exogenous porphyrin fostered NCTC susceptibility to irradiation, resulting in 100% bacterial suppression. (3) Exogenous FMN or FAD, which strongly absorb 450 nm light, did not promote the antimicrobial effect of PBL, neither did exogenous NAD or NADH, two weak blue light-absorbing photosensitizers. Conclusion: These results strengthen our previous assertion that an endogenous chromophore with the capacity to absorb and transform light energy into a biochemical process that engenders bacterial cell death, is essential for 450 nm PBL to suppress GBS.