Sporeless mutants of Bacillus thuringiensis

Five sporeless mutant strains of Bacillus thuringiensis were selected after treatment with the mutagen N-methyl-N′-nitro-N-nitrosoguanidine. Two mutant strains were derived from alesti and three from aizawai. All mutants were completely lacking in ability to form spores. Bipyramidal crystalline bodies of the mutants were very regular in shape, as seen with the parent strains, and lay free in the culture broth with autolysis. The insecticidal activity of mutants was, in principle, the same as that of original strains.Cells of the mutants tended to autolyze easily at the end of cultivation. However, 1–10% of cells still remain living. They are completely killed by heat treatment, e.g., 60°C for 30 min, which, however, causes a slight but nonsignificant reduction of toxicity.Thus, use of these sporeless mutants of Bacillus thuringiensis as microbial insecticide having no viable cells is suggested. They may serve as a biochemical starting material for β-endotoxin.