Improvement of Bacillus thuringiensis delta-endotoxin production by overcome of carbon catabolite repression through adequate control of aeration

Over-production of delta-endotoxin by Bacillus thuringiensis, strain BNS3, entomopathogenic towards lepidoptera through overcome of catabolic repression of its synthesis, was investigated into full-controlled 3 l fermenter using simple and complex media. Equilibrium between density of the vegetative cells and their ability to synthesize toxins during sporulation was shown to be necessary to take into account. By application of various dissolved oxygen profiles during fermentation, it was clear that the level of dissolved oxygen saturation in the fermentation broth affected cell density as well as delta-endotoxin synthesis. Indeed, the adequate dissolved oxygen profiles to be followed throughout the fermentation for high production of bioinsecticides were determined using the two different media. It was found out that aeration rates corresponding to 60% and 70% oxygen saturation during the first 6 h of fermentation should be applied into 15 g l−1 glucose-based medium and 42 g l−1 gruel-based one, respectively. Then, 40% oxygen saturation should be ensured up to the end of fermentation, independently of the carbon source origin. With higher oxygen saturation values, cell densities were increased, but delta-endotoxin synthesis yields were strongly reduced. Cells produced with low aeration into the medium acquired higher capacities to synthesize toxins during sporulation. Interestingly, by following adequate oxygen profiles, it was possible to use increased initial glucose or gruel concentrations, without significant decrease of delta-endotoxin production yields. Consequently, the adequate control of dissolved oxygen in the culture media of B. thuringiensis allowed at least partial overcome of the carbon catabolite repression which seemed to be an apparent regulation, mostly due to the rate of use of energy generated during a first step of high growth of B. thuringiensis cells. These results are of great importance from practical point of view, since it could be possible to produce large quantities of insecticidal crystal proteins during large-scale fermentation, contributing to the reduction of B. thuringiensis insecticides production cost. Interestingly, in bioinsecticides produced with adequate aeration profiles, spores have lower yield, which is in favor of the dissemination of less spores into the environment.