Bt Research 2025, Vol.16, No.3, 103-109 http://microbescipublisher.com/index.php/bt 104 2 Bt Toxin Characteristics and Environmental Limitations 2.1 Structure and mode of action of Bt Cry proteins. The Cry protein of Bt (Bacillus thuringiensis) belongs to the δ -endotoxin family and is often present in the spores of bacteria in crystalline form. It generally has three structural regions: Region I is responsible for perforation, and regions II and III help bind to specific receptors in the insect intestine. When insects consume Cry protein, it dissolves in the alkaline environment of the intestinal tract and is cleaved and activated by proteases. Subsequently, Cry proteins bind to receptors on the surface of intestinal epithelial cells (such as cadherin proteins), create holes, cause cell rupture, and eventually lead to insect death (Soberon et al., 2007). Different types of Cry proteins act on different types of insects and have different toxicity ranges due to their different structures (Deist et al., 2014; Apirajkamol et al., 2025). 2.2 Factors affecting stability The stability of Bt toxins in the environment can be affected by many conditions. The physicochemical properties of the soil (such as pH, temperature, and organic matter content), microbial activities, ultraviolet radiation, and the binding mode with soil materials all affect the degradation rate and persistence of Cry protein (Zhou et al., 2015; Ge et al., 2023). High temperature and ultraviolet rays can cause Bt protein to be inactivated more quickly, while organic matter in the soil and some minerals (such as goethite) can allow Bt protein to stay in the environment for a longer time through adsorption, but this may also reduce its activity. In addition, microorganisms in the soil and gut can break down Bt protein through the action of enzymes, thereby affecting its fate (Ge et al., 2023; Wang et al., 2024). The nutritional status of insects themselves can also have an impact. For example, when they are well-nourished, their resistance to Bt toxin will increase (Deans et al., 2017). 2.3 Current formulations (e.g., microencapsulation, additives). To make Bt toxins more stable in the environment and more persistent in the field, many preparations and formulas have been designed. Methods such as microencapsulation, micronization, and nano-material encapsulation can protect Bt protein from rapid destruction by ultraviolet rays and environmental conditions, thereby prolonging its action time (Zhou et al., 2015). In addition, ultraviolet absorbers, anti-degradation proteins can also be added, or used together with other insecticidal proteins (such as the "gene pyramid" strategy). These methods can all delay the development of pest resistance and enhance the control effect (Deist et al., 2014; Aswathi et al., 2024). Protein engineering methods, such as domain exchange or site-directed mutagenation, can also alter the structure of the Cry protein, making it more effective against specific insects and more adaptable to the environment. 3 Strategies for Enhancing Bt Environmental Stability 3.1 Genetic engineering approaches Genetic engineering is often used to modify Bt toxin genes or develop new hybrid and mutant Bt proteins. This can make the Bt protein more stable and also be effective against more pests. For instance, by using molecular techniques and biotechnology, researchers have obtained some Bt toxins that are more durable in the environment and have a wider range of insecticidal effects. Sometimes, the insecticidal genes of other organisms (such as protease inhibitors, lectins, cholesterol oxidase and chitinase) are expressed together with Bt genes. This can enhance the insecticidal ability and also delay the development of pest resistance. However, when genetically engineered Bt crops and strains are released into the environment, attention should also be paid to gene diffusion and the possible effects on non-target organisms (Azizoglu et al., 2023). 3.2 Microbial engineering approaches The focus of microbial engineering is to modify the Bt strain itself, enabling it to survive longer in the environment and express more toxins. For instance, researchers adjusted the metabolic pathways of the Bt strain to enable it to adapt better in different environments and continuously produce insecticidal proteins (Azizoglu et al., 2023). In addition, there are some "pyramid" type Bt strains that can simultaneously produce multiple insecticidal proteins. This type of strain is more persistent in the field and can also delay pest resistance. These methods laid the foundation for the long-term application of Bt as a biopesticide.
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