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This study was carried out to determine the persistence of toxicity of fermentation extracts of  Bacillus thuringiensis  var.  israelensis  after more than three decades of storage. For this purpose, a population of  Aedes aegypti  was established. The mortality rate of 20 spore-crystal extracts purified using the acetone-lactose coprecipitation method was measured and evaluated by bioassays according to a modified WHO protocol. The extracts with the highest mortality rate were determined in triplicate by their LD 50  and LD 98 . All extracts showed toxicity at the highest tested dose (1000 ppm) and some, such as strains 3260 and 3501, still killed larvae at doses as low as 0.01 ppm. These data are surprising because no study on the activity of  B. thuringiensis  toxic proteins after such a long storage time has been reported.

Persistence of Toxic Activity of Fermentation Extracts fromBacillus thuringiensisvar.israelensisafter More Than Three Decades of Storage