Cell Integrity Markers forin VitroEvaluation of Cytotoxic Responses to Bacteria-Containing Commercial Insecticides

Toxicity of two commercial “BT” products, containingBacillus thuringiensissubsp.kurstakispores (Btk) and associated parasporal inclusion body proteins, was testedin vitrousing two unrelated lepidoperan cell lines and several markers of cell integrity (morphology, quantification of loss of adherence and electron transport (redox) activity, and degradation of nuclear DNA, actin, hsp-70, and β-tubulin). With doses of 10−7, 10−5, and 10−3International Units (IU)/target cell, these markers measured exposure-dependent effects closely linked to cell death, which occurred rapidly once Btk spores germinated, unless inhibited by antibiotic. Derivation of marker half-lives (HL50) revealed that temperature critically affected product performance. Between 34 and 37°C, HL50was <5 hr, but dose discrimination between 10−5and 10−3IU was poor. At temperatures less than 34°C, the resolution between different HL50s and doses increased in a manner directly relating to published data obtained fromin vivoBT-spore-induced LD50assays. It was concluded that BT product toxification is complex, essentially enabled by an autobiotransformation process in which dose–response lag is affected by temperature-dependent temporal expression of spore germination and critical buildup of vegetative cells and byproduct toxicants. Thein vitrodosimetry assays described here are potentially useful for obtaining mechanistic toxicologic data andin vivorelevant quantifications of subingredient activities in various commercial BT formulations as well as in other microbe-based biotechnology products.