Parameters influencing potency of Bacillus thuringiensis var. israelensis products

Bioassays of products based on Bacillus thuringiensis var. israelensis have been carried out according to standard protocols. These analyses revealed that the slopes of the log-probit transformed concentration mortality curves of various products were different from that of the international standards (IPS82 for B. thuringiensis israelensis). For statistical reasons, this invalidates the tests. Products giving various slopes of the concentration mortality curves will obtain different potencies when estimated at a LC90 level than when estimated at LC50 level, as normally done. The LC90 level is probably more relevant for the field effect. Changing the median particle size of a product in a non destructive way results in change of slope and LC50 and thereby potency. Therefore, potency of a product as measured in these bioassays is not just a measure of the quantity of B. thuringiensis israelensis crystal protein present, but a function of product parameters like median particle size. Biochemical methods for quantification of toxin can therefore not relate simply to potency of the products obtained with this method. It is suggested that standard protocols for bioassay may be changed to assure equal particle size of products and samples to obtain parallel dose response.