Abstract
The effect of batch and fed-batch fermentation on the cultivation performance of Bacillus thuringiensis was investigated using a 5-l stirred tank bioreactor. Significantly higher viable cell count (> 1.5 × 1012 CFU/ml) was obtained in the fed-batch compared to batch fermentation (1.4 × 1012 CFU/ml). Glucose feeding during the fermentation seemed to enhance cell growth but failed to enhance the sporulation rate. It was found that sporulation and δ-endotoxin synthesis in fed-batch fermentation could be enhanced by the application of optimal dissolved oxygen tension (DOT) control strategy without affecting the cell growth. Fed-batch cultivation with feeding at the exponential growth phase where the DOT was switched from 80 to 40% at 12 h of cultivation recorded the highest spore count of 7.1 × 1011 spore/ml. Cultures obtained from batch cultivation, as well as fed-batch cultivation with feeding at lag or exponential growth phase and the application of optimal DOT control strategy, recorded the presence of δ-endotoxin; however, none was detected in intermittent fed-batch fermentation. Bioassay data against the bagworm Metisa plana Walker (Lepidoptera: Psychidae) recorded the highest corrected mortality (80%) at 7 days of treatment (DAT), using the culture obtained from fed-batch cultivation with feeding during the exponential growth phase, and the DOT was switched from 80 to 40% at 12 h of cultivation. It is important to note that all cultures containing δ-endotoxin exhibited 100% mortality towards M. plana at 14 DAT.
Highlights
Bacillus thuringiensis (Bt) is widely used to control insect pests in the order Lepidoptera, Diptera, and Coleoptera (Yury et al 2019)
Results from this study demonstrated that the feeding strategy during fed-batch cultivation is crucial and greatly influenced the synthesis of δ-endotoxin
The highest corrected mortality (80%) at 7 days of treatment (DAT) and 14 DAT (100% mortality) was recorded by the culture obtained from fed-batch cultivation with feeding during the exponential growth phase, and the dissolved oxygen tension (DOT) was switched from 80 to 40% at 12 h of cultivation (Table 3)
Summary
Bacillus thuringiensis (Bt) is widely used to control insect pests in the order Lepidoptera, Diptera, and Coleoptera (Yury et al 2019). This bacterium produces spores that contain a proteinaceous body known as crystal protein or δ-endotoxin that possesses insecticidal properties. The bagworm species known as M. plana was classified as the most economically significant insect pest of oil palm (Basri et al 1988). In 2018, the total hectarage of oil palm infested areas, especially in the smallholdings, reached up to more than 30,000 ha, and the use of Bt based biopesticides has been the best alternative to control the pest. The cultivation performance was evaluated in terms of final cell concentration, percentage of sporulation, δ-endotoxin synthesis, and its toxicity towards M. plana
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