Bacillus thuringiensis grows vegetatively when nutrients are abundant, but forms a dormant endospore and one or more crystals when food become scarce. The crystal, with a bipyramidal shape, is composed of the large amount of insecticidal protein toxins known as δ-endotoxins, which account for more than 30% of the total amount of protein made by Bt. Additionally Bt has other insecticidal proteins called Vips (Vegetative insecticidal proteins) produced during vegetative development.
Generally, two groups of endotoxins have been distinguished in the crystal including Cry proteins ( crystal δ-endotoxin) and Cyt proteins (cytolysin). However most Bt toxins are Cry proteins, with high specificity for insect cells both in vivo and in-vitro, which gave them a distinctive importance in insect pest control. These proteins are produced as protoxins and need to be activated to become active. Bt strains are distributed worldwide in soil, stored products, insects, and in insect breeding environments. More than thousands Bt strains have been isolated worldwide (Wang, 2003).
The current classification method is based on the nucleotide sequence homology of the toxin genes and insecticidal activity spectrum of the protein. The Cry genes have been classified into 40 families according to their amino acid sequence similarities (Samsonov et al. 1997, Crickmore et al. 1998) and the sequence of more than 160 Cry genes are known (Arrieta et Al. 2004). The Cry1 gene was the first to be identified in 1990’s and used into corn and cotton to provide protection against lepidopteran insects. However one Bt strain can carry more than one type of Cry gene and thus be effective against a number of different insects species. Moreover the endotoxin genes are located in the bacteria’s plasmids, suggesting a possible natural recombination of toxins among bacteria through transformation via plasmids.