Bioinsecticide activity, bacterial cell lysis and proteolytic activity in cultures of Bacillus thuringiensis subsp. israelensis

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Abstract
Pattern of bioinsecticide activity and cell lysis were monitored in a range of media. Maximum cell lysis, usually exceeding 90% of total cells, occurred at or after the point of maximum bioactivity. In the majority of media tested, insecticide synthesis and cell lysis phases occurred between 24 and 48 h, although they were between 12 and 24 h in some media and later in others. In some media a decline in insecticidal activity was observed after maximum bioactivity was reached. Bioactivity produced per 109 spores ranged from 0.10 to 0.23 mg of purified insecticidal crystal and 3.4 to 5.8 times 104 international toxic units. When proteolytic activity was monitored in cell cultures, extracellular endoprotease was observed before cell lysis and, in addition, there was a linear rise in activity during cell lysis. Aminopeptidase activity appeared in cell free supernatant fluids only during the cell lysis stage. Optima pH for endoprotease and aminopeptidase were 8.5 and 10.25 respectively. The decline in bioinsecticide activity observed may be due to proteolytic action.