Optimization of the Culture Medium, Fermentation Process, and Effectiveness of a Biopesticide from an Iranian Bacillus thuringiensis var. tenebrionis (BN2)

The local strain of B. thuringiensis var. tenebrionis-BN2 (Btt-BN2) was used to control the alfalfa weevil. Experimental design using Response Surface Methodology (RSM) was applied for the optimization of the culture medium and fermentation parameters in order to achieve a high level of Colony-Forming Unit (CFU) (spore mL−1). The parameters including the amount of carbon and nitrogen sources at three levels were investigated with CFU (spore mL−1) response for two sets of experiments. The obtained results showed that the maximum CFU (spore mL−1) for the minimum concentration of oat (2 g L-1), the minimum concentration of corn steep liquor (10 g L-1), and the maximum concentration of sugarcane molasses (10 g L-1) were equivalent to 1.4×1013 spore mL-1. Optimum fermentation parameters to obtain the highest value of CFU (spore mL−1) were determined as a maximum level of pH of eight and a medium level of temperature (280C). Amazingly, optimum conditions enhanced the CFU value to 8.06× 1013 spore mL-1, which is very significant in the Btt research. Finally, the bioassay analysis of Btt in a single system and binary system (combination of two insecticides; Matrine® and Abamectin®) at different concentrations illustrated 83% of mortality efficiency (3 ppm of Btt and 0.5 ppm Matrine) on the 3rd day of treatment and 100% efficiency almost for all combination of Btt with Matrine and/or Abamectin after the 7th day. The bioassay results showed promising environmentally friendly mortality efficiency compared to the current chemical treatments.