One of the approximately 30 subspecies groupings within the approximately 20,000 different strains of the soil bacteria known (collectively) as Bacillus thuringiensis (B.t.). When eaten (e.g., as part of a genetically engineered plant), the protoxin proteins produced by B.t. kurstaki are toxic to certain caterpillars (Lepidoptera larvae), such as the European corn borer (pyralis).