Studies on Bacillus thuringiensis H-14 strains isolated in Egypt—V. Composition and toxicity of the mosquitocidal parasporal inclusions

Abstract

Parasporal inclusions of Bacillus thuringlensis H-14 strains M1 and S128 were characterized by solubilization, electron microscopy, polyacrylamide gel electrophoresis, amino acid analysis and insecticidal activity. Inclusions of both strains are composed largely of protein with 8 to 9% carbohydrate. Amino acid analysis of the purlfied inclusions revealed that the two strains produce inclusions that are closely related to each other but significantly different from lepidopteran-toxic B. thuringiensis parasporal crystals. The LC₅₀ values of the purlfied inclusions of strains M1 and S128 were 3.4 and 2.9 ng/ml, respectively, for fourth instar larvae of Aedes aegypti. Inclusions from strain M1 were resolved into two inclusion bands on the basis of their densities possibly formed as a result of disruption of some envelopes during sonication. Both inclusion types contained proteins of approximately 27, 38 and 66 kDa. The heavlest and more predominant type had an envelope and was either spherical or irregular being composed of several subunits which varied in shape, size and staining densities. The LC₅₀ value was 2.2 ng/ml and the major protein was of approximately 27 kDa. The lightest inclusions type did not have an envelope and showed clear crystal lattices. They were 10 times less toxic to A. aegypti larvae, as compared to the heavy-type inclusions and contained major protein of approximately 66 kDa.