RELIABILITY OF CALCEIN ACETOXY METHYL-ESTER AND ETHIDIUM HOMODIMER OR PROPIDIUM IODIDE FOR VIABILITY ASSESSMENT OF MICROBES

An esterase indicator, calcein acetoxy methyl ester, was used in combination with ethidium homodimer or propidium iodide to evaluate its applicability for viability assays of eukaryotic and prokaryotic microorganisms. These fluorescent probes were previously proven useful in determining the viability of mammalian cells in culture but were not evaluated for their reliability in testing various microorganisms. The fluorescence intensity, lack of bleaching and insensitivity to differences in pH that are characteristic of calcein acetoxy methyl ester enable routine color photomicrographic documentation at magnifications high enough to observe cell morphology for proper identification of the microorganisms. There was good correlation of staining and viability when the method was applied to protozoans. In contrast, the method was not reliable for assessment of the yeast and bacterial cells tested.