In vitro activity of methylene blue on Mycobacterium tuberculosis complex isolates

Methylene blue is used for bacterial staining in microbiology and as an antidote drug in medicine. In this study, we aimed to investigate the antimicrobial effects of methylene blue against Mycobacterium tuberculosis complex clinical isolates. Seventeen stored M. tuberculosis complex clinical isolates were included in the study. The isolates were inoculated into Mycobacteria Growth Indicator Tubes and incubated in Automated Mycobacterial Detection System. Mycobacteria Growth Indicator tubes containing methylene blue at critical concentrations of 0.2, 2, 20, 1000 µg ml-1 and control tube were prepared. Antimicrobial susceptibility testing was performed using Automated Mycobacterial Detection System which is gold standard for second line anti-tuberculosis drug testing. At the end of the study, six clinical isolates were susceptible to methylene blue at all critical concentrations. Five isolates were susceptible to only 1000 µg ml-1 methylene blue. Three isolates were susceptible to 1000 and 20 µg ml-1 methylene blue. Susceptibility rate was found as 94% when the critical proportion was accepted 400 GU (1/100 of control). Significant relationship was observed between the administered methylene blue concentrations and bacterial survival rate in statistical analysis. We conclude that methylene blue may become a potential anti-tuberculosis agent due to its well-known side effects and dosing regimens.