Evidence of the effect of δ-endotoxin ratio in Bacillus thuringiensis crystals on the toxicity against Ephestia kuehniella

The effect of Bacillus thuringiensis cry gene copy-number on δ-endotoxin production and on toxicity has been investigated. When a wild-type strain of B. thuringiensis kurstaki, BNS3, was transformed with the recombinant plasmids pHTcry1Aa, pHTcry1Ac, and pHTcry2Aa, δ-endotoxin synthesis was enhanced by 269, 267, and 243%, respectively. In contrast, spore counts were decreased by at least 50% which was attributed to the cloning vector effect. When B. thuringiensis Cry1Ac or Cry2Aa crystal contents were increased, the recombinant crystals were two- to threefold as toxic as the wild-type crystals against Ephestia kuehniella, demonstrating the importance of δ-endotoxin ratio in the conception of optimal B. thuringiensis bioinsecticide formulation.