Bt Research 2025, Vol.16, No.5, 182-193 http://microbescipublisher.com/index.php/bt 186 (Martínez-Zavala et al., 2020). Genomic studies have shown that Bacillus is generally rich in secondary metabolite synthesis gene clusters, and about 5%~8% of the genes in Bt strains are specifically used for the synthesis of secondary metabolites (Yılmaz et al., 2024). Some Bt strains can also produce secondary metabolites with color, such as melanin, flavin, etc. These pigment molecules are not direct insecticide factors, but can confer special functions to bacteria. For example, melanin can improve the tolerance of spores and toxin crystals to ultraviolet rays, thereby increasing the stability of Bt preparations in the field. In addition, Bt also produces enzymes such as chitinase and protease, which can degrade biological macromolecules around insect epidermis or bacterial bodies, and obtain nutrients in habitat. These enzymes are strictly classified proteins rather than "small molecule" secondary metabolites in the classical sense, but their expression usually occurs in the late sporulation stage or when the host nutrient is exhausted, so they can also be regarded as a type of secondary functional product in the late Bt life history (Figure 2). Figure 2 (A) Microscopic view of Bt SY49.1 after spore-staining. Arrows indicate the endospores. (B) Scanning electron micrograph of the different types of crystal proteins and spores (S) produced by Bt SY49.1. B, bipyramidal; C, cubic; Sp, spherical. (C) Gene synteny of the candidate insecticidal genes identified in the Bt SY49.1 genome (Adopted from Yılmaz et al., 2024) 4.2 The role of secondary metabolites in insect lethality The pathogenic effect of Bt on insects is not entirely completed by Cry toxin alone, and multiple secondary metabolites play an auxiliary role at each stage of the infection process. During the initial infection phase, enzyme products secreted by Bt may help break through the insect surface barrier. The midgut of insect larvae has a perimeter composed of chitin and protein. The chitinase secreted by Bt can locally degrade the perimeter, promoting better access to toxins and acting on intestinal epithelial cells. Insects may experience secondary sepsis after the toxins work and cause cells to rupture intestinal walls. At this time, the overgrowth of Bt itself or symbiotic bacteria is one of the important factors that cause host death (Grizanova et al., 2022). The antibacterial peptide secondary metabolites produced by Bt can inhibit some
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