Abstract
Antimicrobial blue light (aBL) has attracted increasing interest for its antimicrobial properties. However, the underlying bactericidal mechanism has not yet been verified. One hypothesis is that aBL causes the excitation of intracellular chromophores; leading to the generation of reactive oxygen species (ROS) and the resultant oxidization of various biomolecules. Thus, monitoring the levels of redox-sensitive intracellular biomolecules such as coproporphyrins, as well as singlet oxygen and various ROS may help to uncover the physiological changes induced by aBL and aid in establishing the underlying mechanism of action. Furthermore, the identification of novel targets of ROS, such as fatty acids, is of potential significance from a therapeutic perspective. In this study, we sought to investigate the molecular impact of aBL treatment on methicillin-resistant Staphylococcus aureus (MRSA). The results showed that aBL (5–80 J/cm2) exhibited a bactericidal effect on MRSA, and almost no bacteria survived when 80 J/cm2 had been delivered. Further studies revealed that the concentrations of certain intracellular molecules varied in response to aBL irradiation. Coproporphyrin levels were found to decrease gradually, while ROS levels increased rapidly. Moreover, imaging revealed the emergence and increase of singlet oxygen molecules. Concomitantly, the lipid peroxidation product malondialdehyde (MDA) increased in abundance and intracellular K+ leakage was observed, indicating permeability of the cell membrane. Atomic force microscopy showed that the cell surface exhibited a coarse appearance. Finally, fatty acid profiles at different illumination levels were monitored by GC-MS. The relative amounts of three unsaturated fatty acids (C16:1, C20:1, and C20:4) were decreased in response to aBL irradiation, which likely played a key role in the aforementioned membrane injuries. Collectively, these data suggest that the cell membrane is a major target of ROS during aBL irradiation, causing alterations to membrane lipid profiles, and in particular to the unsaturated fatty acid component.
Highlights
As a novel light-based disinfection approach, antimicrobial blue light, in the wavelength range of 405–470 nm, has attracted increasing interest due to its intrinsic antimicrobial effect
It was of significance to examine the changes in the levels of intracellular molecules such as coproporphyrin, reactive oxygen species (ROS) and lipid oxides, which are thought to be critical for the mechanism of action of antimicrobial blue light (aBL) inactivation
More and more research has indicated that aBL is effective in the photoinactivation of antibiotic-resistant bacteria. aBL was similar to photodynamic therapy, while need not the addition of exogenous photosensitizers
Summary
As a novel light-based disinfection approach, antimicrobial blue light (aBL), in the wavelength range of 405–470 nm, has attracted increasing interest due to its intrinsic antimicrobial effect. Compared to traditional photodynamic therapy, aBL therapy excites the endogenous chromophores of bacteria, and does not require the addition of exogenous photosensitizers. In comparison to ultraviolet irradiation, aBL shows much less detrimental effects in mammalian cells. The bactericidal activity of aBL is non-specific, and many microbial cells, including various antibiotics resistant strains, are highly sensitive to this treatment. ABL therapy has previously shown promise as a treatment for various clinical pathogens, such as Pseudomonas aeruginosa, Acinetobacter baumannii, methicillinresistant Staphylococcus aureus (MRSA), and Candida albicans (Dai et al, 2012). In the case of S. aureus, the bactericidal effect has been reported to be highly efficient. The survival rate of US300 MRSA (5 × 106 CFU/ml cultures) was reduced by 96% under 405 nm aBL irradiation at 60 J/cm (Bumah et al, 2013), and survival fraction of MRSA USA300 (7 × 106 CFU/ml cultures) was decreased by 98% under irradiation at 60 J/cm (Bumah et al, 2015)
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