s usually bipyramidal in shape. Upon lysis of the mother cell, both the spore and the inclusion separate and are released into the environment (i.e., the soil, ponds, streams) (6:364). Within these areas, the larvae of mosquitos and blackflies feed on small particulate matter. Ingestion of B. thuringiensis' inclusion is followed by midgut solubilization and the concomitant release of toxic proteins. These "protoxins" are then activated by larval proteases. The activated toxins disrupt the larval midgut epithelium's structural integrity. Inevitably, this sequence of events leads to insect death (3:615).
The incapacitation of its host insect provides B. thuringiensis with several survival benefits. In addition to ingesting the bacteria's toxic crystalline inclusion, insect larvae typically ingest the spore as well. The biochemical events which kill the insect provide the bacteria with a unique ecological niche. B. thuringiensis is essentially a soil microorganism. Not only is the insect hemolymph an excellent nutrient environment for bacterial proliferation, but it also provides "a rare opportunity for soil microorganisms to react concentrations sufficient for genetic exchange via cell mating (2:19)."
Bacillus thuringiensis' precise entomopathogenic mode of action is rather complex. There are several different B. thuringiensis subspecies, and a variety of susceptible hosts. In addition, the bacteria are known to produce more than one toxin; moreover, these different proteins may interact both with each other and with the spore to produce toxicity. Finally, the different hosts can additionally possess multiple toxin receptor sites (2:18).
Of the different toxic substances produced by B. thuringiensis, perhaps the most potentially pathogenic to insects consist of ¯-exotoxin and (-endotoxin. Beta-exotoxin is an adenine nucleotide capable of inhibiting RNA synthesis. However, this particular nucleotide toxin i...