Rapid fluorometric quantification of bacterial cells using Redsafe nucleic acid stain

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. Redsafe fluorometric assay was evaluated in comparison with MTT colorimetric assay as a colou- rimetric assay for enumeration of bacterial cells. 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. 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.