Statistical Analysis of Cultural Parameters Influencing Delta-Endotoxins and Proteases Productions by Bacillus thuringiensis kurstaki

Abstract

Bacillus thuringiensis is a spore-forming bacterium that produces delta-endotoxins, which are toxic to lepidopterans, dipterans and coleopterans. Concomitantly with production of delta-endotoxins, B. thuringiensis produces proteolytic enzymes during growth and sporulation, affecting yields of toxins production. In this work, an experimental factorial design was employed for the dual evaluation of effects of delta-endotoxins and proteolytic enzyme on activity of a B. thuringiensis subsp. kurstaki strain. Main effects and interactions of three factors (soybean meal, starch and aeration) under two conditions (0 and 7 g/l NaCl) were analysed using statistical techniques. Regression models, known as useful tool to build relationships between variables and subsequently to make predictions, were suggested to fit the experimental data. Yields ranged between 6.10–15.76 × 10−7 μg and 18.18–30.30 × 10−7 IU for delta-endotoxin and proteolytic enzymes per cell, respectively. When NaCl (7 g/l) was added to the medium, yields ranged from 15.51 to 38.06 for delta-endotoxins and from 9.56 to 48.73 for proteases. Starch, soybean meal and aeration improved the delta-endotoxins production. Interactions (starch × air) and (soybean meal × air) showed a negative effect on delta-endotoxin yield, while (starch × soybean meal) had no effect. The active function of aeration was enhanced when NaCl (7 g/l) was added, and effects of all interactions on delta-endotoxins yield had been nullified. Soybean meal and aeration had negative effects on proteolytic activity, in contrast to starch, (starch × air) and (soybean × air), which improved proteolytic activity yields. The inhibitory role of soybean meal shifted to positive effect according to the presence of NaCl (7 g/l) in the culture medium.