In vitro Antibacterial Activity of Dye Compounds

Background: Methylene blue and some of its analogues have known antibacterial activity, however their exact mechanism of action is unknown. Objective: In this study, the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of several methylene blue analogues were determined against five bacterial strains, whereafter the data were used to create and validate a pharmacophore model. Methods: The agar dilution method was used to screen the analogues for antibacterial activity, while the broth microdilution method was used to determine their MIC and MBC. A pharmacophore model was constructed and validated using the rank score, fit value, enrichment factor (EF10%), hit rate (HR10%) and receiver operating characteristic area under the curve (ROC-AUC) as metrics. Results: Against Staphylococcus aureus, pyronin B (0.125 mu g/ml) was more active than tetracycline (1 mu g/ml) and pyronin Y (0.5 mu g/ml), 1,9-dimethylmethylene blue (2 mu g/ml), basic blue 3 (2 mu g/ml), new methylene blue (2 mu g/ml) and Nile blue (2 mu g/ml) had similar activity compared to tetracycline. Pyronin B, 1,9-dimethylmethylene blue and new methylene blue were bactericidal. A pharmacophore model was created (rank score: 36.55, max. fit value: 3), which was able to identify active analogues out of the test set (EF10%: 2.83, HR10%: 28.57%, ROC-AUC: 0.84 +/- 0.04). The pharmacophore model highlighted that a positive ionisable, aromatic ring as well as a hydrophobic moiety are important for antibacterial activity. Conclusion: Methylene blue analogues were found to have potent antibacterial activity and a pharmacophore model was created to understand the structural requirements for activity.