Standardization of Bacillus thuringiensis Products: Homology of the Standard

Abstract

THE lethality of Bacillus thuringiensis to many lepidopterous larvae has led to its development as a “biological insecticide”. Products based on this bacterium contain two active ingredients, the spore and the crystal of toxic protein, and sometimes a third, the exotoxin. The water-soluble exotoxin is secreted into the medium during vegetative growth and the crystal is formed alongside the spore at sporulation. Because the crystal is by far the most important ingredient against many Lepidoptera and cannot be estimated chemically, the realization that the insecticidal activities of different preparations cannot be consistently measured by counting the spores was an important advance in the problem of standardization. Bioassay, with species of insect sensitive only to the crystal, is taking the place of the spore count. The recent adoption of one batch of stable bacterial preparation, E61, as the international standard has provided both a recognized material with which other materials can be compared and a basis for establishing toxicity units. Such measurement of the crystal content of materials should work well with products containing the same strain of bacterium as the standard, that is, when the standard is homologous.