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The discovery of antibiotics is one of the key achievements of modern medicine. Antibiotic use has contributed enormously to human health. However, microorganisms can evolve and adjust to changes in their environment and have subsequently developed several protective mechanisms to reduce their susceptibility to antibiotics 1. More and more potent antibiotics lose their efficacy over time, rendering them useless for conditions they could once successfully treat 2-4. Therefore, there is an urgent need for screening and development of new antibacterial agents effective against resistant strains 5-8. Screening of active substances plays a crucial role in drug research and development. The 3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide (MTT) assay has been widely adopted for low cost, rapidity and high throughput format 9. In this assay, the yellow and water-soluble MTT is converted to the purple and water insoluble MTT formazan in the presence of mitochondrial dehydrogenase as shown in Scheme-I 10. As a result, the amount of formazan dye produced is directly proportional to the number of metabolically active cells and indicates the reducing potential of the cells 11. The MTT assay uses frequently, while the reported conditions and parameters of the assay vary widely. Thus attempts to modification of MTT assay are never ceased 12-15. Incubating cells with MTT,