Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 7 6 2016 02 01 319 323 EN Ehsan Khalili Department of Clinical Biochemistry, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. Vahid Hosseini Department of Clinical Biochemistry, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. Roya Solhi Department of Clinical Biochemistry, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. Mahdi Aminian Department of Clinical Biochemistry, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran AND Recombinant Vaccine Research Center, Tehran University of Medical Sciences, Tehran, Iran. 2015 11 21 2015 12 08 Background and Objectives: Numerous procedures in biology and medicine require the counting of cells. Direct enumer- ation of Colony Forming Units (CFUs) is time-consuming and dreary accurate cell counting on plates with high numbers of CFUs is error prone. In this study we report a new indirect cell counting method that was developed based on the use of Redsafe fluorometric assay. The usefulness of Redsafe, a nucleic acid stain, in liquid medium is based on the binding of the fluorescent dye to DNA. Materials and Methods: Redsafe fluorometric assay was evaluated in comparison with MTT colorimetric assay as a colou- rimetric assay for enumeration of bacterial cells. Results: Obtained results showed that fluorometric assay threshold for LB grown E. coli is 6×104  CFU/ml. Redsafe fluo- rescent assay can be used as a rapid and inexpensive method for bacterial enumeration and quantification with increased sensitivity. Conclusion: The sensitivity of the Redsafe fluorometric assay for detection and enumeration of bacterial cells was 2-log-unit more than that was observed for the MTT assay. https://ijm.tums.ac.ir/index.php/ijm/article/view/808 https://ijm.tums.ac.ir/index.php/ijm/article/download/808/535