Bt Research 2025, Vol.16, No.5, 182-193 http://microbescipublisher.com/index.php/bt 183 pathways of Bt and its relationship with insect pathogenicity. First, introduce the main characteristics of Bt strain metabolism, including sugar metabolism, protein and amino acid metabolism, lipid metabolism, etc.; then explore the coupling relationship between Bt crystal toxin synthesis and metabolic pathways; analyze the role of Bt's secondary metabolites in the pathogenic process and the function of the metabolic regulatory network; take Bt's prevention and control of corn borer as a case, and deeply analyze the metabolic mechanism of Bt under specific insect exemption, including Cry1Ab toxin synthesis support and genetic engineering of Bt corn. Through the above content, we hope to deepen our understanding of the biochemical basis of Bt pathogenicity and provide theoretical support for improving the application effect of Bt biopesticides. 2 Main Metabolic Characteristics of Bt 2.1 Sugar metabolism pathways and their energy supply As a heterotrophic bacteria, Bt uses organic matter such as carbohydrates as the main carbon source to obtain energy and intermediate metabolites through central metabolic pathways such as glycolysis (EMP pathway), tricarboxylic acid cycle (TCA cycle) (Wang et al., 2013). Under vegetative conditions, Bt cells break down glucose through the EMP pathway to power and generate mature metabolites through the TCA cycle to meet growth needs. When entering late growth or environmental stress, Bt significantly adjusts the sugar metabolism pathway to adapt to environmental changes. Studies have shown that under alkaline stress, the glycolysis-related genes of Bt are greatly increased, producing a large number of metabolic acids such as lactic acid and malic acid, neutralizing the alkaline environment to help bacteria survive. The accumulation of these organic acids is not only a means for Bt to cope with external stress, but may also indirectly affect its pathogenic process. Sugar metabolism is also closely related to spore production and toxin synthesis. Adequate sugar supply can improve the growth rate and energy reserves of bacteria in the logarithmic phase, but excessively available carbon sources may delay spore production through carbon repression effects, thereby delaying the formation of crystal toxins. Bt cells often begin spore production and synthesize toxins when they enter a stable phase, which is usually accompanied by depletion of sugar sources and reprogramming of metabolic flows (Xie et al., 2019). Therefore, suitable sugar metabolism levels must not only meet the rapid growth in the early stage, but also smoothly transition to toxin synthesis during the spore production period. 2.2 Regulatory effects of protein and amino acid metabolism The need for nitrogen sources in Bt is equally important during growth and spore formation. Protein and amino acid metabolism not only provides structural and functional components, but also affects the expression of virulence-related genes through global regulatory effects. Adequate nitrogen source (amino acid) is conducive to rapid proliferation and accumulation of reserves of bacteria, but excess nitrogen source may inhibit the opening of spore production and toxin genes. This is because the presence of global regulators of nutritional induction in Gram-positive bacteria, such as CodY protein, remains active when amino acid and GTP levels are high, thereby suppressing the transcription of secondary metabolism and sporogenesis-related genes (Qi et al., 2015). When nutrients are exhausted, CodY is inactivated, relieving the inhibition of spore production and toxin synthesis pathways, allowing the bacteria to enter the stage of spore production toxin synthesis. It is expected that there is an amino acid sensing mechanism similar to B. subtilis in Bt. High concentrations of branched chain amino acids can affect the expression balance of genes related to toxin synthesis through regulatory proteins such as CodY. In addition, some specific amino acids are precursors to toxin synthesis. Bt also shows certain ability in decomposing environmental proteins, secreting proteases to degrade proteins in insect carcasses or culture medium to obtain nitrogen sources. However, excessive efflux protease activity in early stages of infection may be detrimental to the toxin's function. 2.3 Lipid metabolism and cell membrane function Lipids are an important part of the cell membrane and cell wall, and are related to the changes and stability of membrane structure when spores and toxin crystals are formed. Bt provides raw materials for cell membrane phospholipids through the fatty acid synthesis pathway during the vegetative growth stage, and can use part of the carbon flow to synthesize energy storage substances when carbon is too high. When entering the spore production
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