Bt Research 2025, Vol.16, No.5, 224-233 http://microbescipublisher.com/index.php/bt 224 Review Article Open Access Impact of Plasmid Loss on the Virulence of Bacillus thuringiensis MingLi Tropical Microbial Resources Research Center, Hainan Institute of Tropical Agricultural Resources, Sanya, 572025, Hainan, China; Corresponding email: ming.li@hitar.org Bt Research, 2025, Vol.16, No.5 doi: 10.5376/bt.2025.16.0025 Received: 26 Aug., 2025 Accepted: 09 Oct., 2025 Published: 25 Oct., 2025 Copyright © 2025 Li, This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Preferred citation for this article: Li M., 2025, Impact of plasmid loss on the virulence of Bacillus thuringiensis, Bt Research, 16(5): 224-233 (doi: 10.5376/bt.2025.16.0025) Abstract Bacillus thuringiensis is an important microbial insecticide and is widely used in agricultural pest control. Bt can produce insecticidal crystal protein (Cry) and lipopeptide toxin (Cyt) that are highly virulent to various pests such as Lepidoptera, Coleoptera, Diptera, and a variety of toxins such as the vegetative insecticidal protein (Vip). Most of these toxin genes are located on plasmids and are the molecular basis of Bt virulence. Bt strains usually carry multiple plasmids, and the virulence genes loaded on different plasmids constitute an important determinant of their insecticidal profile. The presence of plasmids in Bt plays a critical role in insecticidal virility and strain adaptability. This study analyzed the types and mechanism of action of Bt insecticidal toxins encoded by the plasmid, the functional characteristics and stability mechanism of Bt plasmid, as well as the molecular mechanism of plasmid loss and its impact on Bt virulence, and summarized the detection and analysis methods and coping strategies of plasmid loss. By sorting out recent research progress, we emphasize that the impact of proofreading the law of plasmid loss on Bt virility is of great significance to improving the stability of Bt preparations and guiding the construction of Bt engineering strains. Keywords Bacillus thuringiensis; Plasmid loss; Insecticidal toxin; Toxicity; Plasmid stability 1 Introduction Bacillus thuringiensis (Bt) is a Gram-positive Bacillus that has become the most widely used microbial insecticide because it can produce consporin crystal toxins that have a toxic effect on specific insects. Currently, Bt preparations account for the vast majority of the global biopesticide market and play an irreplaceable role in agricultural and forestry pest control. Bt strains synthesize insecticidal crystal protein (δ-endotoxin) during spore formation, which can dissolve and release toxins in the high alkaline environment of insect midgut, and have a highly specific pathogenic effect on hundreds of pest larvae such as Lepidoptera, Diptera, Coleoptera, etc. At the same time, Bt strains usually carry multiple plasmids that encode insecticidal toxin genes and other functional genes, which are the genetic basis of Bt insecticidal effect. Plasmids play many roles in Bt strains. On the one hand, virulence genes such as cry, cyt, and vip carried on the plasmid directly determine the virulence level and virulence spectrum of the strain to the target insect; on the other hand, the plasmid may also carry auxiliary functional genes such as antibiotics and stress resistance, affecting the adaptability of the strain inside and outside the host (Cheliah et al., 2019). However, plasmid DNA is generally less stable than chromosomes and is prone to loss or rearrangement in the absence of selection pressure. This means that during long-term passage or environmental survival, the Bt strains may gradually lose their plasmids, resulting in a loss of virulence-related genes or a decrease in expression (Nair et al., 2021). Plasmid loss will weaken the insecticidal effect of Bt strains and reduce their bio-defense application value. Therefore, it is of great significance to explore the effect of plasmid loss on Bt virulence. It not only helps to deeply understand the molecular basis of Bt virulence formation, but also provides theoretical guidance for improving the genetic stability of Bt preparations and delaying the virulence decay in field strains. Based on the research progress in the past five years, this study reviews the molecular basis of Bt virulence, the functional characteristics of Bt plasmids, the mechanism of plasmid loss and its impact on virility, and looks forward to strategies to prevent plasmid loss to maintain or enhance Bt virulence.
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