Bt Research 2025, Vol.16, No.6, 269-277 http://microbescipublisher.com/index.php/bt 273 5.2 Expression and dissemination of antibiotic and heavy metal resistance genes on plasmids In addition to harsh environments, Bt also has to deal with the "competitive situation" in the microbial world. At this time, the resistance genes on the plasmid become particularly important. Many plasmids of Bt strains carry antibiotic and heavy metal resistance elements. For instance, the giant plasmid poh1 not only contains antimicrobial peptides but also a complete set of resistance gene clusters (Masri et al., 2015). These things not only enable BTS to survive longer in contaminated areas or highly competitive environments, but also allow them to "share resources" among different bacteria through the horizontal transfer function of plasmids (Meric et al., 2018). Once this gene flow is activated, resistance traits spread rapidly among populations, and the survival resilience of Bt also increases accordingly. Simply put, no matter how rapidly the environment changes, the plasmid system can help it "temporarily equip itself". 5.3 Role of plasmids in biofilm formation and ecological niche expansion of Bt To gain a firm foothold in the environment, Bt cannot rely solely on its "insecticidal ability"; it also needs to see if it can settle down for a long time - at this point, biofilm becomes the key. The ability of Bt to form biofilms is often related to plasmids, especially plasmids like pHT8_1. The Rap-Phr system carried on them can flexibly regulate the generation pathways of spores and biofilms (Fazion et al., 2018). Sometimes these systems may suppress, and sometimes they may promote. It all depends on whether Bt "wants to live alone" or "is preparing to move into a host". Once the biofilm is successfully formed, the environmental tolerance and colonization ability of Bt will both reach a new level, and its adaptability will also be enhanced accordingly (Gillis and Mahillon, 2014). Not to mention, in soil or insects, plasmid conjugation and transfer are highly active, which further accelerates the diffusion rate of beneficial genes. All these indicate that plasmids are not merely "functional components", but more like the "behind-the-scenes commanders" of Bt in the process of adapting to complex environments. 6 Case Studies: Plasmid-Driven Metabolic Traits in Specific Bt Strains 6.1 Functional analysis and application of Cry plasmids in Bt kurstaki Among Bt strains, the significance of Cry toxin needs no elaboration, and the Kustak subspecies is a representative one. Its insecticidal ability, in the final analysis, still depends on the δ -endotoxin genes carried on those plasmids. However, these plasmids are not always the more the better. For instance, some studies have found that after appropriately deleting certain plasmid fragments, the toxin production actually increased and the proteolytic activity also became stronger (Driss et al., 2011). This may sound like "subtractive optimization", but it actually indicates one thing: not all plasmids are adding points; some may even hold them back. So some people began to think: Could the combination of plasmids be adjusted to specifically retain those parts that are most beneficial for toxin synthesis? From the perspective of biological insecticides, this does indeed provide new ideas for the optimization of Bt strains. 6.2 Plasmids related to mosquito larvicidal Activity in Bt israelensis When it comes to killing mosquitoes, the Bt strain of the Israeli subspecies is a seasoned expert. The pBtoxis plasmid in its body carries multiple toxin genes that are lethal to mosquito larvae and is the "main force" in combating mosquitoes. However, somewhat surprisingly, the QBT220 strain isolated from Qatar lacks two common genes - cry10A and cyt1C - in its pBtoxis plasmid, yet its insecticidal ability does not decrease but increases instead (Nair et al., 2021). This is thought-provoking - perhaps some toxins are actually "competing for resources" rather than simply piling up. Some combinations after plasmid rearrangement can actually stimulate more efficient toxin expression. Such observations indicate that the role of the plasmid system in Bt strains is not fixed, and its structural adjustment can bring about unexpected insecticidal performance. 6.3 Comparative studies on plasmid-mediated metabolism in Bt strains from different ecological niches It is no longer new that the plasmid configuration of Bt strains grown in different environments varies. Take the H3 plant in Lebanon for example. It was isolated from the soil and carried three large plasmids. One of them also carried 11 Cry toxin genes, along with some functional enzymes such as chitinase and cellulase (Fayad et al.,
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