Isolation of Bacillus thuringiensis (Bt) from Soil sample


Bacillus thuringiensis (Bt) is a gram-positive, soil-dwelling bacterium that naturally produces a toxin that is fatal to certain herbivorous insects. Bt has been used as a biological pesticide since the 1920s and is commonly used in organic farming. Bt also occurs naturally in the gut of caterpillars of various types of moths and butterflies, as well as on leaf surfaces, aquatic environments, animal feces, insect-rich environments, and flour mills and grain-storage facilities.

Bt has many strains or subspecies that affect different insect groups, such as beetles, flies, mosquitoes, black flies, and moths. Each strain produces a specific type of protein crystal (delta endotoxin) during sporulation that binds to specific receptors on the membranes of mid-gut cells of the targeted insects, resulting in their rupture and death. Other organisms that lack the appropriate receptors in their gut are not affected by the Bt toxin and therefore are not harmed by Bt.

Isolation of Bt from soil samples involves a series of steps to select and identify Bt strains from the diverse bacterial population present in the soil. The main principle of isolation is to use a thermal shock treatment followed by selective germination of spores on nutrient agar plates. Thermal shock eliminates all bacteria that cannot produce endospores from the soil sample, while selective germination allows only spore-forming bacteria to grow on the plates. Further tests are then performed to confirm the presence of Bt based on its morphology, staining properties, catalase activity, growth temperature range, and parasporal crystal formation.

In this article, we will explain the procedure for isolation of Bt from soil samples in detail and discuss the expected results of this method. We will also highlight the importance and applications of Bt as a biopesticide for insect pest management.