Bacillus thuringiensis var. israelensis δ-endotoxin: Evidence of neurotoxic action

The neurotoxic, cytolytic, and lethal properties of a 25-kDa protein isolated from the δ-endotoxin of Bacillus thuringiensis subsp. israelensis (Bti) were compared with those of the alkaline-dissolved whole extract of the δ-endotoxin and the cytolytic enzyme phospholipase A 2. In situ electrophysiological measurements of the ventral nerve cord of larval Trichoplusia ni treated at 15 and 25°C with various dosages of the three protein preparations showed that only the whole extract of the δ-endotoxin induced hyperexcitation. When the appearance of lactate dehydrogenase activity in the haemolymph of toxin-injected larvae was used to quantitate cell lysis, the cytolytic 25-kDa protein was found to be a potent cytotoxic factor like phospholipase A 2, whereas cytotoxicity of the whole δ-endotoxin extract was relatively low. Bioassays at 28, 17, and 9°C demonstrated a positive temperature correlation with mortality for the whole toxin extract, but showed inverse correlations for the two cytolytic materials. This and previous studies indicate that insect mortality is not solely due to the cytolytic activity of the Bti δ-endotoxin upon injection into T. ni. An unidentified neurotoxic factor(s) exists within the Bti δ-endotoxin which induces hyperexcited activity in the insect nervous system. This and/or other factors impart the δ-endotoxin with biological and physiological activities different from the isolated cytolytic component.