Quantitative bioreduction assays for calibrating spore content and viability of commercial Bacillus thuringiensis insecticides

The redox dyes MTT (3-{4,5-dimethylthiazol-2-yl}-2,5-diphenyl tetrazolium bromide, thiazolyl blue) and XTT (sodium 3′- {1-[(phenylamino)-carbonyl]-3,4-tetrazolium}-bis{4-methoxy-6-nitro} benzene sulphonic acid hydrate) were used as dosimetry reporters in liquid (multi-well) and solid support (membrane) assays to estimate spore viability and content of commercial BT products derived from fermentation of Bacillus thuringiensis subsp kurstaki (Btk) and subsp israelensis (Bti). QC tests on five BT products were done using spore, protein and gene contents, and morphology (scanning electron microscopy) as indicators. Spore levels (6–40 × 10⁹ colony forming units (CFU) ml⁻¹) were approximately equivalent when based on International Units (IU) of potency. Spore viability was highly stable over a broad range of temperatures and pHs but germination and growth were restricted (optima: pH ≈ 7.5 and 37°C). Quantitative bioreduction activity (QBA) of MTT and XTT correlated with vegetative cell production. Depending on manipulation of pre-assay conditions, both dyes could discriminate doses from ∼2 to 10⁹ spores (or 10⁻³ to 10⁶ IU). Non-toxic effects of XTT and its formazan product enabled automated collection of data on growth and dose. Solid support assays also reliably estimated product dosage by in situ detection of CFU. With appropriate reference dilutions of microbe-containing products the QBA assays can provide high throughput QC monitoring of product comparisons and field release in aerial spray or water injection applications. Received 18 December 1996/ Accepted in revised form 11 March 1997