A rapid colorimetric assay to evaluate the effects ofBacillus thuringiensis toxins on cultured insect cells

Summary A new cytotoxicity assay has been developed to assess the activity of the toxins produced byBacillus thuringiensis kurstaki on CFl cells. The assay is performed in a 96-well plate using a tetrazolium salt, MTT, as the substrate. The compound diffuses through the cell membrane and is reduced by mitochondrial dehydrogenases of live cells to a purple color compound, which precipitates within the cells. The precipitate is dissolved by isopropanol and the absorbance at 570 nm is measured with a 96-well plate reader. The amount of dye converted is proportional to the number of liver cells and the length of incubation with substrate. The LC₅₀ of the 60 kDa toxin, an activated form of the 130 kDaB. t. kurstaki toxin has been determined and found to be comparable to the data obtained by the trypan blue staining of dead cells.