The Mode of Action of Bacillus Thuringiensis Endotoxins

The use of synthetic organic insecticides developed during the last half of this century may pose risks to human health and can cause environmental prob­ lems. Consequently, interest has developed in using alternative strategies for insect pest management. One contemporary approach that has received atten­ tion is the development of Bacillus thuringiensis (Bt) toxins as insecticides. B. thuringiensis, a gram-positive bacteria, produces a proteinaceous paraspor­ al crystalline inclusion during sporulation. Upon ingestion by insects, this crystalline inclusion is solubilized in the midgut, releasing proteins called S-endotoxins. These proteins (protoxins) are activated by midgut proteases, and the activated toxins interact with the larval midgut epithelium causing a disruption in membrane integrity and ultimately leading to insect death. In spite of significant efforts directed towards the study of Bt, its use in pest control is restricted, in part because of the selectivity of Bt and in part because of its moderate efficacy. Attempts to improve these two qualities have met with only moderate success because inadequate effort has been devoted to understanding the molecular basis of selectivity and insecticidal properties of these toxins.