Activity of delta-endotoxin protein crystals in Bacillus thuringiensis mutants when influenced by UV treatments

The ability of some isolates of Bacillus thuringiensis to produce dark brown pigment was measured as an indicator to UV resistance. M₅ as an indigenous Egyptian isolate was used as wild type to improve its resistance to UV. It was exposed to UV irradiation for different periods ranging between 1 and 10?h. The induced mutants were examined morphologically by phase contrast microscope. One hundred and forty four mutants were obtained; 10 of them were selected and tested for their toxicity against Spodoptera littoralis. The results showed that mutants 62, 65 and 85 were the most toxic ones. These three mutants and the wild type were examined by transmission electron microscope. Crystal proteins with bipyramidal shape and active against Lepidopteran insects were detected in all the selected mutants.