Why Bacillus thuringiensis insecticidal toxins are so effective: unique features of their mode of action

The spore-forming bacterium Bacillus thuringiensis produces intracellular i nclusions comprised of protoxins active on several orders of insects. These highly effective and specific toxins have great potential in agriculture a nd for the control of disease-related insect vectors. Inclusions ingested b y larvae are solubilized and converted to active toxins, in the midgut. The re are two major classes, the cytolytic toxins and the delta -endotoxins. T he former are produced by B. thuringiensis subspecies active on Diptera. Th e latter, which will be the focus of this review, are more prevalent and ac tive on at least three orders of insects. They have a three-domain structur e with extensive functional interactions among the domains. The initial rev ersible binding to receptors on larval midgut cells is largely dependent up on domains II and III. Subsequent steps involve toxin insertion into the me mbrane and aggregation, leading to the formation of gated, cation-selective channels. The channels are comprised of certain amphipathic helices in dom ain I, but the three processes of insertion. aggregation and the formation of functional channels are probably dependent upon all three domains. Letha lity is believed to be due to destruction of the transmembrane potential, w ith the subsequent osmotic lysis of cells lining the midgut. In this review , the mode of action of these delta -endotoxins will be discussed with emph asis on unique features. (C) 2001 Federation of European Microbiological So cieties. Published by Elsevier Science B.V. All rights reserved.