INTRODUCTION TO B.t

Bacillus thuringiensis (Bt), is a gram-positive soil bacterium. The prevalence of this strain is not restricted and has been isolated worldwide from many habitats, including soil, stored-product dusts, and insects, deciduous and coniferous leaves. Bacillus thuringiensis forms parasporal crystals called as cry proteins during the stationary phase of its growth cycle. These crystals are specifically toxic to certain orders and species of insects, like Lepidoptera, Diptera, and Coleoptera. Many different strains of Bt have been shown to produce these inclusions of insecticidal crystal protein (ICP

During sporulation, it synthesizes a cytoplasmic inclusion containing one or more proteins that are toxic to insect larvae. Upon completion of sporulation the parent bacterium lyses to release the spore and the inclusion. In these inclusions, the toxins exist as inactive protoxins. When the inclusions are ingested by insect larvae, the alkaline pH solubilizes the crystal. The protoxin is then converted in to an active toxin after processing by the host proteases present in the midgut.

It has been indicated that the activated toxin binds to insect-specific receptors exposed on the surface of the plasma membrane of midgut epithelial cells and then inserts into the membrane to create transmembrane pores that cause cell swelling and lysis and eventually death of the insect.

Due to their high specificity for these unique receptors on the membrane of the gut epithelial cells, these toxins (delta-endotoxins) are harmless to non-target insects and the end-user i.e man are compatible with integrated pest management programs.