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 againstCandida 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 ofMycobacteriumat 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 inMycobacterium. Mechanistic insights revealed that MB causes energy dependent membrane perturbation in bothC. albicansandMycobacterium. 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 inMycobacterium. We provided evidence that although biofilm formation was disrupted in both organisms, cell adherence to human epithelial cells was inhibited only inMycobacterium. 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,CandidaandMycobacterium,which can be extrapolated to improve our understanding of finding novel therapeutic options.