Bt Research 2025, Vol.16, No.2, 47-54 http://microbescipublisher.com/index.php/bt 49 will reduce the expression of plasmid genes and the burden of plasmids, thus making it easier to stably maintain multidrug-resistant plasmids (Kloos et al., 2021). In addition, plasmids and chromosomes can also share genes, allowing resistance genes to spread rapidly through horizontal transfer (Wang et al., 2021; Wein et al., 2021). Under antibiotic pressure, the transcriptional regulation of plasmids and chromosomal genes works in synergy to promote bacterial growth, drug resistance and biofilm formation, which makes it easier for Bt and other bacteria to colonize and survive in complex environments (Zhang et al., 2021). Figure 1 Characterization of antibiotic resistant and metabolic genes (Adopted from Palomino et al., 2022) 4 Ecological and Evolutionary Roles of Plasmid Diversity 4.1 Horizontal gene transfer and its role in population adaptability Plasmids are the main tools for horizontal gene transfer (HGT) between bacteria. They can rapidly spread some important genes among different species and strains, such as genes for antibiotic resistance, metal tolerance and degradation of new substrates. This genetic exchange enables bacterial populations to adapt to environmental changes more quickly (Wiedenbeck and Cohan, 2011; Smalla et al., 2015; Petersen et al., 2019). In Bt and its closely related species, pathogenic genes, stress-resistant genes and regulatory systems (such as the Rap/Phr system) on plasmids can spread through HGT. This can enhance the survival and reproduction of the strain in insect hosts and complex environments (Perchat et al., 2024). Meanwhile, the diversity of plasmids provides more genetic variations for the population and also enhances the overall stability to the environment (Heuer and Smalla, 2012; Finks et al., 2024).
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