Bt Research 2024, Vol.15, No.3, 118-130 http://microbescipublisher.com/index.php/bt 119 2 Overview of Bt Isolates and Habitats 2.1 Common habitats of Bt isolates Bacillus thuringiensis (Bt) is a ubiquitous bacterium found in a variety of habitats worldwide. Commonly, Bt isolates are oBtained from soil, plant surfaces (phylloplanes), and insect guts. For instance, a study conducted in Bangladesh identified Bt isolates from vegetable and crop-cultivated soils, phylloplanes, and insect guts, with the highest prevalence in soil samples (Shishir et al., 2012). Similarly, research in Qatar revealed a diverse collection of Bt isolates from soil, highlighting the soil as a rich source of Bt diversity (Figure 1) (Nair et al., 2018). In Iran, Bt strains were isolated from fields, gardens, and desert and semi-desert areas, further emphasizing the adaptability of Bt to different environmental conditions (Rashki et al., 2021). 2.2 Environmental factors influencing Bt distribution The distribution of Bt isolates is influenced by various environmental factors, including soil composition, climate, and the presence of host insects. For example, the diversity of Bt strains in Qatar was attributed to the unique soil ecology of the region, which supports a wide range of Bt isolates with different crystal morphologies and endotoxin profiles (Nair et al., 2018). In Bangladesh, the prevalence of Bt in soil samples compared to leaf and insect samples suggests that soil properties and agricultural practices may play a significant role in Bt distribution (Shishir et al., 2012). Additionally, the presence of specific insect hosts can influence the distribution and diversity of Bt isolates, as seen in the study of Brazilian Bt isolates, where genetic diversity was linked to the ability to target Aedes aegypti larvae (Fernandes et al., 2021). 2.3 Collection and isolation techniques The collection and isolation of Bt isolates involve several techniques to ensure the recovery of diverse and representative samples. Common methods include selective culturing, molecular characterization, and bioassays. In Bangladesh, selective methods were used to oBtain Bacillus cereus-like isolates, which were then identified as Bt based on hemolytic activity, parasporal crystal proteins, and plasmid profiles. In Qatar, scanning electron microscopy was employed to analyze the crystal forms of Bt isolates, revealing a high abundance of spherical crystals (Shishir et al., 2012). Molecular techniques such as 16S rDNA gene sequencing and PCR amplification are also used to confirm the identity of Bt strains and characterize their genetic profiles (Shishir et al., 2012; Rashki et al., 2021). Advanced techniques like pulsed field gel electrophoresis (PFGE) are utilized to separate and identify plasmid profiles in Bt strains. PFGE is particularly effective for separating high molecular weight plasmid DNAs, which are difficult to resolve using conventional gel electrophoresis. This method was successfully applied to analyze plasmid profiles in 10 Bt strains, providing detailed information on the number and size of plasmids (Zhou et al., 2014). Additionally, techniques like amplified fragment length polymorphism (AFLP) and repetitive element polymorphism (Rep-PCR) are used to assess genetic variability and molecular markers among Bt isolates (Valicente and Silva, 2017). Figure 1 Electrophoresis gel showing seven different plasmid patterns observed among the Bt collection (1–7) (Adopted from Nair et al., 2018) Image caption: L represents a 1 kb plus ladder; H14 is the reference strain Bacillus thuringiensis israelensis, HD1 is the reference strain Bacillus thuringiensis kurstaki; 1, QBt229; 2, QBt6; 3, QBt43; 4, QBt212; 5, QBt99; 6, QBt3; 7, QBt375 (Adopted from Nair et al., 2018)
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