1. Strain improvement of Bacillus thuringiensis

Objective: 1. To study and characterize B.thuringiensis SP41  
                 2. To engineer B.thuringiensis for enhancement of insecticidal activity

          B. thuringiensis is a gram positive bacterium which has a capacity to produce crystalline inclusions comprised of protoxins. It harbors several large and small plasmids. The large plasmids encode protoxin or crystal (Cry) proteins. Based on flagella antigen serotyping, over 70 species of B. thuringiensis have been identified which will be activated to active toxin by protease in the midgut of insect larvae. The toxin binds specifically with receptor of epithelial cells in insect midgut.

          B. thuringiensis subsp. aizawai (Bta) strain SP41 produces very large crystal. It is highly toxic against Spodoptera exigua and S. litura. The expression of several cry genes as well as many genes involved in cry gene expression was studied at transcriptional and translational levels. In other Bta strain BTA1, which harbors fewer cry genes and produce smaller size crystal, strain improvement by co-expression of chitinase gene at spore-stage is studied. To ensure stability, integration of different chitinase gene cassette into its chromosome is performed and it is demonstrates that toxicity is enhanced several fold. Chitinase was proofed to disintegrate peritrophic membrane of insect larvae.

           For more information on subspecies identification and Cry protein classification, please see at http://www.lifesci.sussex.ac.uk/home/Neil_Crickmore/Bt/

           In each subspecies of B.thuringiensis, they produce different types of Cry proteins. Some Cry proteins such as Cry1Aa, Cry1Ab, Cry1C are toxic to Lepidoptera larvae. Some subspecies produce Cry protein toxic to mosquito larvae (dipteran) such as Cry4A, Cry4B and Cry11A, whereas Bt. subspecies which produce Cry3A is highly toxic to Coleoptera larvae.

Figure 1: Scanning electron micrograph of spores and crystals of B.thuringiensissubspecies aizawai SP41.
Figure 2: B.thuringiensis SP41 showing spores and crystals.
Figure 3: Mechanism of toxin action