[Transgenic bioinsecticides inimical to parasites, but imical to environment]

Identification of Bacillus thuringiensis (Bt) parasporal crystalline inclusions composed of Cry proteins (=delta-endotoxins) resulted in introduction of microbial pesticides for biological control of some parasites. Delta-endotoxins are encoded by cry genes and are active against pest and nuisance insects (mostly mosquitoes and black flies--vectors of still important infectious diseases). The recent significant progress in DNA recombination technique may overcome limitations (a short residual persistence and a narrow spectrum of activity) associated with application of Bt conventional products. An introduction of cry genes from mosquitocidal subspecies B. th. israelensis (Bti) to the aquatic microorganisms inhabiting the same water bodies as mosquito and fly larvae (Diptera), has considerably improved the toxin delivery system to target insects. However, in the first experiments, in which Bti genes were cloned in cyanobacteria (Agmenellum quadruplicatum, Synechocystis PCC6803), a low gene expression was observed. Thus, it was necessary to integrate cry genes with strong promoters or to increase the number of vector-introduced copies. To overcome the obstacles of low gene expression and regulatory restriction for recombinant organisms, Bti spore/crystal formulations were encapsulated in the aquatic protozoan, Tetrahymena pyriformis. Large numbers of crystals (180 to 240/cell) were accumulated in its food vacuoles. This system resulted also in an increase in toxin persistence from 24 to 71 h. Cloning Bti genes in B. sphaericus (which also produces mosquitocidal proteins) was another way of an increasing Bt crystal residual activity. In this case, the crystals were additionally protected by B. sphaericus exosporium. These transgenic bacteria produced large amounts of delta-endotoxins that remained under water surface longer than the wild B. sphaericus strains. Moreover, they had a broader spectrum of insecticidal activity, because B. sphaericus is toxic mostly to Culex and Anopheles, and Bti--mostly to Culex, Aedes and some Simmulidae. Gram-negative bacteria (Asticcacaulis excentricus, Caulobacter crescentus and Ancylobacter aquaticus) turned out also to be effective delta-endotoxin producers. They grow on simple media and do not contain proteases which could degrade Cry proteins. In some cases, 100% mosquito larvae mortality was observed as a result of an exposure to transgenic microorganisms containing Bti genes. However, transgenic techniques are still not very popular in the world, despite their efficacy in biological control of insects. The transgenic organism construction is expensive and time-consuming. Genetic engineering is still raising a lot of anxieties and doubts concerning inappropriate use of modified organisms. On the other hand, this technology could solve many problems associated with vectors of important diseases, which are still unapproachable to contemporary medicine.