Effect of the volumetric oxygen mass transfer coefficient on producing δ-endotoxins by Bacillus thuringiensis in culture medium based on forage palm

Abstract

Bacillus thuringiensis is the main commercial source of safe insecticidal toxins. On the industrial scale, oxygenation is the most important variable for enhancing the production of δ-endotoxin by B. thuringiensis and improving its toxicity. Therefore, the parameters of growth kinetics, the formation of spores, the production of δ-endotoxin, and its toxicity against Spodoptera frugiperda were evaluated in batch cultures of B. thuringiensis cultivated in a culture medium based on forage palm with different KLα values. In all conditions, diauxic growth curves were observed due to the change of carbon source. Maximum cell concentration increased when KLα values were raised from 11 to 16 h−1, but biomass productivity was enhanced only when KLa was increased from 11 to 13 h−1. Due to the diauxic growth, spore production and δ-endotoxin production were observed at two different times. The first peak was generally lower than the second peak. At both peaks, a high concentration of spores was reached using a KLa of 13 h−1, but the highest production of δ-endotoxin and the maximum toxin yield based on spores or substrate were observed in the KLa of 16 h−1. Therefore, a bioassay against S. frugiperda was performed using δ-endotoxin produced under a KLa of 16 h−1. Although more δ-endotoxin was produced in the second peak than in the first peak, the highest toxicity was observed with toxins produced in the first peak. These results suggest that the KLα affects the production of δ-endotoxin and that toxicity against S. frugiperda decreases when fermentation time is increased.