CHAPTER ONE
INTRODUCTION
Bacillus thuringrensis (Bt) is a well known and widely studied bacterium which is known for its use in pest management. Today it is the most successful commercial xenobiotic with its worldwide application when compared with the chemical pesticides; Bacillus thuringiensis has the advantages of being biologically degradable, selectively active on pests and less likely to cause resistance. Safety of Bacillus thuringiensis formulations for humans, beneficial animals and plants explains the replacement of chemical pesticides in many countries with these environmentally friendly pest control agents.
Bacillus thuringiensis was first isolated by the Japanese Scientist Ishiwata (1901) from skilkworm larvae, bombyxmori, exhibiting sotto disease. After 10 years, Berliner (1911) isolated the square gram (+) positive, spore-forming, rod shaped soil bacterium from disease flour moth larvae, Anngasta Kachmiccalla, in the Thuringia region of the Germany and named it as Bacillus thuringiensis.
In the early 1930s Bacillus thuringiensis was used against Ostrinianubilis, the European corn borer. The first commercial product was available in 1938 in France, with the trade name sporeine (Weiser, 1986). It was Bacillus thuringiensis subspecies Kurstaki that was used for the control of the insect (Lepidopteran) pests in agriculture and forestry (Luthy & Ebersold, 1981). New commercial products arrived in 1980s after the discovering of subspecies thuringiensis opened the gate for black fly and mosquito larvae control.
Like all organisms, insect are susceptible to infection by pathogenic microorganisms, many of these infections agents have a narrow host range and therefore, do not cause uncontrolled destruction of beneficial insects and are not toxic to vertebrates. Bacillus thuringiensis is a major microorganism, which shows entamopathogenic activity (Glazer & Nikaido, 1995, Schnepf, et al. 1998) which forms parasporal crystals during the stationary phase of its growth cycle.
Most Bacillus thuringiensis preparations available on the market contain spores with parasporal inclusion bodies composed of δ – endotoxins. In commercial production, the crystals and spores obtained from fermentation are concentrated and formulated for spray on application according to conventional Agriculture practices (Baum, Kakefuda, & Gawron-Burke, 1996). There are many strains of Bacillus thuringiensis having insecticidal activity against insect order (eg Lepidoptera, Diptera, Homoptera, Mollaphage, Coloptera). Only a few of them have been commercially developed.
Bacillus thuringiensis insecticides are divided into three groups, group one has been used for the control of lepidopterans. These groups of insecticides are formulated with Bacillus thuringiensis Subspecies. Kurstaki, group two contains thesandiego and tenebrionis strains of Bacillus thuringiensisand has been applied for the control of certain celopterans and their larvae. Group three contains the Israelensis strains of Bacillus thuringiensis which has been used to control black flies and mosquitoes.
CRYSTAL COMPOSITION AND MORPHOLOGY
The existence of parasporal inclusions in Bt was first noted I 1915 (Berliner 1915) but their protein composition was not delineated until the 1950s (Angus 1954). Hannay (1953) detected the crystalline fine structure that is a property of most of the parasporal inclusion. Bacillus thuringiensis subspecies can synthesize more than one inclusion, which may contain different ICPs. ICPs have been called data endotoxins; however since the term endotoxin usually refers to toxin associated with the other membranes of gram-negative bacteria, comprising a core lipopoly saccharide. Depending on their ICP composition, the crystals have various forms (bipyramidal, cuboidal, flat rhomboid, or a composition with two or more crystal types. A partial correlation between crystal morphology, ICP composition, and bioactivity against target insects has been established (Bulla et al.1977). Hofte and Whitely, 1989, Lynch and Baumman, 1985).