Bacillus thuringiensis has been in commercial use as a microbial insecticide for many years. Until recently, it had been assumed that this crystal-forming bacterium was only entomocidal for the agriculturally important Lepidoptera insects. Now, however, a new variety, Bacillus thuringiensis var. israelensis, has been found which is effective against both mosquitoes and black flies. This unexpected observation poses the question how var. israelensis differs from the other B. thruingiensis strains and whether these differences can be further exploited. Information pertaining to these questions should be relevant to an understanding of the molecular basis of host specificity in the microbial insecticides. Since the protein crystal is responsible for the entomocidal activity exhibited by B. thuringiensis, it is the logical starting point for a study on the altered host specificity of var. israelensis. Accordingly, we will compare purified crystals from var. israelensis and selected Lepidoptera-active strains with regard to as many relevant biochemical parameters as possible. Specifically, we will examine: 1/ The influence of crystal morphology (e.g. Amorphous vs. Discrete) on toxicity. 2/ The conditions necessary to solubilize var. israelensis crystals. Solubility is a prerequisite for toxicity. 3/ The presence of chitin barrier in the mosquito larval gut lining. Does var. israelensis have chitinase activity? 4/ The presence and function of interchain disulfide bonds in the var. israelensis crystal. These unusual bonds are the dominant structural features of other B. thuringiensis crystal types. and 5/ The chemical identity of the crystal digestion product which acts as the ultimate mosquito toxin. The purification and characterization of the var. israelensis crystal is underway in our laboratory. Additionally, we have recently demonstrated that the crystal's larval toxicity can be simulated by cyclic peptide ionophores.
