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

Abstract

Significance of methylene blue (MB) in photodynamic therapy against microbes is well established. Previously, we have reported the antifungal potential of MB against Candida albicans. The present study attempts to identify additional antimicrobial effect of MB against another prevalent human pathogen, Mycobacterium tuberculosis (MTB). We explored that MB is efficiently inhibiting the growth of Mycobacterium at 15.62 μg/ml albeit in bacteriostatic manner similar to its fungistatic nature. We uncovered additional cell surface phenotypes (colony morphology and cell sedimentation rate) which were impaired only in Mycobacterium. Mechanistic insights revealed that MB causes energy dependent membrane perturbation in both C. albicans and Mycobacterium. We also confirmed that MB leads to enhanced reactive oxygen species generation in both organisms that could be reversed upon antioxidant supplementation; however, DNA damage could only be observed in Mycobacterium. We provided evidence that although biofilm formation was disrupted in both organisms, cell adherence to human epithelial cells was inhibited only in Mycobacterium. Lastly, RT-PCR results showed good correlation with the biochemical assay. Together, apart from the well-established role of MB in photodynamic therapy, this study provides insights into the distinct antimicrobial mode of actions in two significant human pathogens, Candida and Mycobacterium, which can be extrapolated to improve our understanding of finding novel therapeutic options.

Trehalose Conjugation Enhances Toxicity of Photosensitizers against Mycobacteria

“…Hameed et al showed that methylene blue causes energy-dependent membrane perturbation and DNA damage in M. smegmatis . 30 Hence, it is possible that tethering methylene blue to trehalose hinders the compound from entering cells and consequently abolishes additional intracellular toxicity, as described above.…”

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

“…Methylene blue inhibited in vitro colistin-resistant strains of Acinetobacter baumannii, Mycobacterium ulcerans, Mycobacterium spp. and Candida albicans [6] , [7] , [8] . Methylene blue was also effective in vivo against Buruli ulcer in experimental Mycobacterium ulcerans infection in mice [7] .…”

In Vitro Activity of Methylene Blue on Mycobacterium Tuberculosis Complex Isolates

“…Shim et al reported the inhibitory effect of methylene blue-mediated photodynamic therapy on Mycobacterium smegmatis in 2016 [35] . One year later, Pal et al conducted research on Candida albicans and Mycobacterium smegmatis and stated that MB alone inhibited the growth of Mycobacterium smegmatis at 15.62 µg ml -1 in a bacteriostatic manner similar to its fungistatic characteristic (Myco-bacteria means fungus-like bacteria in Greek) [36] . They also reported that MB was leading to impaired cell surface phenotypes, altered colony morphologies, and DNA damage in Mycobacteria.…”

“…They also reported that MB was leading to impaired cell surface phenotypes, altered colony morphologies, and DNA damage in Mycobacteria. They suggested performing further investigations on Mycobacterium tuberculosis, since this pathogen contains unique cell envelope components with complex lipids providing pathogenicity [36] .…”