Abstract
Antimicrobial resistance is a global, mounting and dynamic issue that poses an immediate threat to human, animal, and environmental health. Among the alternative antimicrobial treatments proposed to reduce the external use of antibiotics is electromagnetic radiation, such as blue light. The prevailing mechanistic model is that blue light can be absorbed by endogenous porphyrins within the bacterial cell, inducing the production of reactive oxygen species, which subsequently inflict oxidative damages upon different cellular components. Nevertheless, it is unclear whether other mechanisms are involved, particularly those that can affect the efficacy of antimicrobial blue light treatments. In this review, we summarize evidence of inherent factors that may confer protection to a selected group of bacteria against blue light-induced oxidative damages or modulate the physiological characteristics of the treated bacteria, such as virulence and motility. These include descriptions of three major photoreceptors in bacteria, chemoreceptors, SOS-dependent DNA repair and non-SOS protective mechanisms. Future directions are also provided to assist with research efforts to increase the efficacy of antimicrobial blue light and to minimize the development of blue light-tolerant phenotypes.
Highlights
CP, coproporphyrin; PP, protoporphyrin; CFU, colony forming units. * Light dosage was calculated by multiplying light intensity (Watt/cm2 ) with treatment time. ** Bactericidal efficacy is defined as the reduction in the number of viable bacteria induced by antimicrobial blue light treatment. *** Number of bacterial cells was quantified by flow cytometry. a Photosensitizer concentrations, light dosages and bactericidal efficacies separated by the word “or” indicate differences between planktonic or biofilm cells
These findings demonstrate the multi-factorial consideration that needs to be evaluated during an antimicrobial blue light treatment of B. subtilis, treatment conditions and possible presence of individual strains with varying stressosome compositions
The complex biological pathways involved in bacterial responses to antimicrobial blue light present challenges to be overcome
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The two most relevant porphyrins are protoporphyrin IX and coproporphyrin (I and III), as their involvements in antimicrobial blue light treatments have been demonstrated in both Gram-positive and Gram-negative bacteria, such as S. aureus, P. aeruginosa, A. baumannii, and Helicobacter pylori [29,30,31,32]. Downstream of ALA, various enzymes are involved in the conversion of one tetrapyrrole structure to another, leading to the formation of different photoactive porphyrins (Figure 2) In addition to their respective functions in the forward reaction of heme biosynthesis, some of these enzymes are involved in feedback regulations, as demonstrated in E. coli, where the overexpression of hemD and hemF resulted in the accumulation of ALA, whereas hemB, hemG and hemH had a negative effect on ALA production [33]. Protoporphyrin IX and uroporphyrin III absorb blue light at approximately 405–420 nm, whereas coproporphyrin III has the highest absorption at approximately 390 nm [40]
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