XTT salt assay for determination of viable count of plague vaccine

The objective was to study the possibility of determination of viable count of plague vaccine by XTT assay and compare the results with that of the conventional viable count of colony-forming unit of the same samples.The principle was that the cleavage of XTT by dehydrogenase enzymes of metabolically active bacterium yields a highly colored water-soluble formazan product and the count of formazan reflected that of bacterium.XTT method was employed for plague vaccine and a linear regression made between the absorbance of formazan and different concentrations of standard plague vaccine.The linear equations were used to read out the viable count of unknown samples by absorbance.Then the results were compared with the conventional viable count of the same samples.The method XTT could show different concentrations of plague vaccine in a certain range.The regression coefficient might reach 0.999 9.The data of viable count of 3 lots of plague vaccine by the XTT assay and colony forming unit(CFU) demonstrated no significant difference(P0.05).The XTT assay was faster for determining viable count of plague vaccine.This assay could be applied for rapid determination of viable count of plague vaccine.