Bt Research 2024, Vol.15, No.3, 118-130 http://microbescipublisher.com/index.php/bt 124 6 Functional Implications of Plasmid Variation 6.1 Role in Bt adaptation and survival Plasmids play a crucial role in the adaptation and survival of Bacillus thuringiensis (Bt) in various environments. These mobile genetic elements enable rapid genetic changes that can confer selective advantages to their bacterial hosts. For instance, plasmids can carry genes that help Bt adapt to different ecological niches by encoding traits such as toxin production, stress tolerance, and nutrient acquisition). The presence of multiple replicons within plasmids, as observed in the COMPASS database, extends the host range and enhances the survival of Bt by allowing it to thrive in diverse environments (Douarre et al., 2020). Additionally, plasmids can harbor genes involved in heavy metal homeostasis, which is essential for Bt to survive in contaminated environments (Krawczyk et al., 2018). This adaptability is further supported by the ability of plasmids to carry genes that facilitate horizontal gene transfer, thereby spreading advantageous traits within bacterial communities (Smalla et al., 2015). 6.2 Impact on Bt virulence and toxin production The virulence of Bt is significantly influenced by the plasmids it harbors. Plasmids can carry virulence-associated genes, including those encoding Cry toxins, which are pivotal for Bt's pathogenicity against insect hosts. Studies have shown that the presence of specific plasmids can enhance the virulence of Bt by increasing the copy number of toxin genes, thereby boosting toxin production (Masri et al., 2015). For example, the co-introduction of plasmids harboring different carbapenemase genes in bacterial hosts has been shown to increase fitness and virulence, suggesting a similar mechanism could be at play in Bt. Furthermore, plasmids can also carry genes that enhance Bt's ability to form biofilms, resist host immune responses, and survive within host organisms, all of which contribute to its virulence (Lee et al., 2020). The genetic diversity of virulence plasmids, as seen in various Escherichia coli strains, underscores the potential for plasmids to drive the evolution of virulence traits in Bt (Hazen et al., 2015). 6.3 Contribution to genetic diversity Plasmids are a major source of genetic diversity in bacterial populations, including Bt. They facilitate the horizontal transfer of genes between different strains and species, leading to the rapid dissemination of beneficial traits. This genetic exchange is crucial for the evolution of new virulent strains and the adaptation to changing environmental conditions (Smalla et al., 2015). The diversity of plasmid-encoded genes, such as those involved in antibiotic resistance, virulence, and metabolic functions, contributes to the overall genetic variability within Bt populations (Wawire et al., 2021). Comparative genomic analyses have revealed that plasmids from different ecological niches exhibit distinct genetic signatures, reflecting the adaptation of Bt to specific environments (Davray et al., 2020). The dynamic nature of plasmid acquisition and loss, as observed in Shiga toxin-producing E. coli, highlights the role of plasmids in shaping the pangenome and virulence factor repertoires of bacterial populations (Nakamura et al., 2020). This genetic plasticity, driven by plasmid variation, is essential for the long-term survival and evolutionary success of Bt in diverse habitats. In summary, plasmid variation in Bt isolates from different habitats has profound functional implications. Plasmids enhance Bt's adaptability and survival by encoding niche-specific traits, increase its virulence through the amplification of toxin genes, and contribute to genetic diversity by facilitating horizontal gene transfer. These factors collectively underscore the importance of plasmids in the ecological and evolutionary dynamics of Bt. 7 Case Studies of Plasmid Profiles in Specific Habitats 7.1 Soil-derived Bt isolates Soil is a rich reservoir for Bacillus thuringiensis (Bt) isolates, which exhibit a diverse range of plasmid profiles and insecticidal properties. For instance, a study conducted in Qatar identified seven distinct plasmid profiles among 700 Bt isolates from soil samples. These isolates displayed a variety of crystal morphologies and endotoxin protein profiles, indicating a high level of genetic diversity. The study highlighted the potential of these isolates for developing novel bio-insecticides targeting different insect orders, such as Dipteran, Lepidopteran, and Coleopteran insects (Nair et al., 2018).
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