Nonphotodynamic Roles of Methylene Blue: Display of Distinct Antimycobacterial and Anticandidal Mode of Actions

In Vitro and Ex Vivo Investigation of the Antibacterial Effects of Methylene Blue against Methicillin-Resistant Staphylococcus aureus

Methylene blue (MB) is a water-soluble dye that has a number of medical applications. Methicillin-resistant Staphylococcus aureus (MRSA) was selected as a subject for research due to the numerous serious clinical diseases it might cause and because there is a significant global resistance challenge. Our main goal was to determine and analyze the antibacterial effects of MB against S. aureus both in vitro and ex vivo to enhance treatment options. A total of 104 MRSA isolates recovered from various clinical specimens were included in this study. Minimum inhibitory concentration (MIC) values of MB against MRSA isolates were determined by the agar dilution method. One randomly selected MRSA isolate and a methicillin-susceptible S. aureus strain (S. aureus ATCC 25923) were employed for further evaluation of the antibacterial effects of MB in in vitro and ex vivo time-kill assays. A disc diffusion method-based MB + antibiotic synergy assay was performed to analyze the subinhibitory effects of MB on ten isolates. MICs of MB against 104 MRSA isolates, detected by the agar dilution method, ranged between 16 and 64 µg/mL. MB concentrations of 4 and 16 µg/mL showed a bactericidal effect at 24 h in the ex vivo time-kill assays and in vitro time-kill assays, respectively. We observed a significant synergy between cefoxitin and methylene blue at a concentration of 1-2 μg/mL in two (20%) test isolates. Employing MB, which has well-defined pharmacokinetics, bioavailability, and safety profiles, for the treatment of MRSA infections and nasal decolonization could be a good strategy.

Increasing the Selectivity of Light-Active Antimicrobial Agents - Or How To Get a Photosensitizer to the Desired Target

Photosensitizers combine the inherent reactivity of reactive oxygen species with the sophisticated reaction control of light. Through selective targeting, these light-active molecules have the potential to overcome certain limitations in drug discovery. Ongoing advances in the synthesis and evaluation of photosensitizer conjugates with biomolecules such as antibodies, peptides, or small-molecule drugs are leading to increasingly powerful agents for the eradication of a growing number of microbial species. This review article, therefore, summarizes challenges and opportunities in the development of selective photosensitizers and their conjugates described in recent literature. This provides adequate insight for newcomers and those interested in this field.

Photodynamic therapy-a promising treatment of oral mucosal infections

The treatment of oral mucosal infections is increasingly challenging owing to antibiotic resistance. Therefore, alternative antimicrobial strategies are urgently required. Photodynamic therapy (PDT) has attracted attention for the treatment of oral mucosal infections because of its ability to effectively inactivate drug-resistant bacteria, completely heal clinical infectious lesions and usually offers only mild adverse reactions. This review briefly summarizes relevant scientific data and published papers and discusses the potential mechanism and application of PDT in the treatment of oral mucosal infections.

Methylene blue inhibits replication of SARS-CoV-2 in vitro

In December 2019, a novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), causing coronavirus diseases 2019 (COVID-19) emerged in Wuhan, China. Currently there is no antiviral treatment recommended against SARS-CoV-2. Identifying effective antiviral drugs is urgently required. Methylene blue has already demonstrated in vitro antiviral activity in photodynamic therapy as well as antibacterial, antifungal and antiparasitic activities in non-photodynamic assays. In this study. non-photoactivated methylene blue showed in vitro activity at very low micromolar range with an EC₅₀ (median effective concentration) of 0.30 ± 0.03 μM and an EC₉₀ (90% effective concentration) of 0.75 ± 0.21 μM at a multiplicity of infection (MOI) of 0.25 against SARS-CoV-2 (strain IHUMI-3). The EC₅₀ and EC₉₀ values for methylene blue are lower than those obtained for hydroxychloroquine (1.5 μM and 3.0 μM) and azithromycin (20.1 μM and 41.9 μM). The ratios C_max/EC₅₀ and C_max/EC₉₀ in blood for methylene blue were estimated at 10.1 and 4.0, respectively, following oral administration and 33.3 and 13.3 following intravenous administration. Methylene blue EC₅₀ and EC₉₀ values are consistent with concentrations observed in human blood. We propose that methylene blue is a promising drug for treatment of COVID-19. In vivo evaluation in animal experimental models is now required to confirm its antiviral effects on SARS-CoV-2. The potential interest of methylene blue to treat COVID-19 needs to be confirmed by prospective comparative clinical studies.

Successful treatment of Pityriasis Versicolor by photodynamic therapy mediated by methylene blue

BACKGROUND

Although systemic therapies are recommended for severe or recalcitrant cases of pityriasis versicolor (PV), they are not free of important side effects and drug interactions. Photodynamic therapy (PDT) utilizes the action of singlet oxygen and free radicals produced by a light-activated photosensitizer to kill viruses, bacteria, or fungi. In this study, the effect of a PDT mediated by methylene blue (MB) in PV was evaluated.

METHODS

Five women with PV disseminated on the back and diagnosed by fresh microscopic analysis were treated with a solution of MB (2%) applied to the PV lesions for 3 minutes. Next, a red LED lamp (λ = 630±5 nm, 37 J/cm² ), placed 100 mm from the skin for 10 minutes, was applied on the dyed PV lesions. Six sessions of MB/PDT were implemented with a 2-week interval in between. Wood's lamp examination was used to monitor fungal infection at each time point.

RESULTS

Complete cure was observed in the five women at the 4 weeks post-treatment follow-up. Fluoresce images from PV lesions by Wood's lamp allowed to evaluate whether the lesions were healed or not at each time point. No patient showed relapse at the 6-month follow-up. The patients did not have any adverse effect, and good cosmetic outcome was observed.

CONCLUSIONS

Six sessions of MB/PDT spaced at 14-day intervals are sufficient for the treatment for PV in healthy patients.

Trehalose Conjugation Enhances Toxicity of Photosensitizers against Mycobacteria

Trehalose is a natural glucose-derived disaccharide found in the cell wall of mycobacteria. It enters the mycobacterial cell through a highly specific trehalose transporter system. Subsequently, trehalose is equipped with mycolic acid species and is incorporated into the cell wall as trehalose monomycolate or dimycolate. Here, we investigate the phototoxicity of several photosensitizer trehalose conjugates and take advantage of the promiscuity of the extracellular Ag85 complex, which catalyzes the attachment of mycolic acids to trehalose and its analogues. We find that processing by Ag85 enriches and tethers photosensitizer trehalose conjugates directly into the mycomembrane. Irradiation of the conjugates triggers singlet oxygen formation, killing mycobacterial cells more efficiently, as compared to photosensitizers without trehalose conjugation. The conjugates are potent antimycobacterial agents that are, per se, affected neither by permeability issues nor by detoxification mechanisms via drug efflux. They could serve as interesting scaffolds for photodynamic therapy of mycobacterial infections.

Understanding lipidomic basis of iron limitation induced chemosensitization of drug-resistant Mycobacterium tuberculosis

Under limited micronutrients condition, Mycobacterium tuberculosis (MTB) has to struggle for acquisition of the limited micronutrients available in the host. One such crucial micronutrient that MTB requires for the growth and sustenance is iron. The present study aimed to sequester the iron supply of MTB to control drug resistance in MTB. We found that iron restriction renders hypersensitivity to multidrug-resistant MTB strains against first-line anti-TB drugs. To decipher the effect of iron restriction on possible mechanisms of chemosensitization and altered cellular circuitry governing drug resistance and virulence of MTB, we explored MTB cellular architecture. We could identify non-intact cell envelope, tampered MTB morphology and diminished mycolic acid under iron restricted MDR-MTB cells. Deeper exploration unraveled altered lipidome profile observed through conventional TLC and advanced mass spectrometry-based LC-ESI-MS techniques. Lipidome analysis not only depicted profound alterations of various lipid classes which are crucial for pathogenecity but also exposed leads such as indispensability of iron to sustain metabolic, genotoxic and oxidative stresses. Furthermore, iron deprivation led to inhibited biofilm formation and capacity of MTB to adhere buccal epithelial cells. Lastly, we demonstrated enhanced survival of Mycobacterium-infected Caenorhabditis elegans model under iron limitation. The present study offers evidence and proposes alteration of lipidome profile and affected virulence traits upon iron chelation. Taken together, iron deprivation could be a potential strategy to rescue MDR and enhance the effectiveness of existing anti-TB drugs.