BIOCONTROL POTENTIAL OF BACILLUS THURINGIENSIS ISOLATED FROM SOIL SAMPLES AGAINST LARVA OF MOSQUITO

chapter one

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).

GENERAL CHARACTERISTICS OF BACILLUS THURINGLENSIS

Bacillus thuringiensis is a member of the genes Bacillus and like the other members of the taxon, has the ability to form endospores that are resistant to inactivation by heat, desiccation and organic solvent. The spore formation of the organism varies from terminal to subterminal in sporangia that are not swollen, therefore, Bacillus thuringiensis resembles other members of Bacillus species in morphology and shape (Stahly, Andrews, & Yousten, 1991). The organism is gram-positive and facultitative anaerobes. The shape of the cells of the organism is rod. The size when grown in standard liquid media varies 3 – 5um.

The most distinguishing features of Bacillus thuringiensis from other closely related Bacillus species. (eg Bacillus anthracis, Bacillus. cereus) is the presence of the parasporal crystal body that is near to the spore outside the exosporangium during the endospore formation, which is shown in figure 1:1 (Andrews, Bibilops, & Bulla, 1985; Andrews, Faust, Wabiko, Raymond, & Bulla, 1987; Bulla, Faust, Andrews, & Goodma, 1995). Bacillus thuringiensis is an insecticide producing variant of Bacillus cereus (Gordon, Haynes, & Pang, 1973) several Bt species also produce Bacillus cereus type

enterotooxin (Carlson, & Kolsto, 1993) plasmids coding for the insecticidal toxin of Bacillus thuringiensis have been transferred into B. cereus to make it a crystal producing variant of Bacillus thuringiensis(Gonzalez, Brown, Carlton, 1982) molecular methods including genomic restriction digestion analysis and 16 rRNA sequence comparison support that Bacillus thuringiensis, Bacillus anthracis and Bacillus cereus are closely relocated species and they should be considered as a single species (Carlson, Caugant, & Kolstra, 1994; Ash , Farrow, Dorsch, Stackebrandt, & Collins. 1991; Helgason et al.2000).

CLASSIFICATION OF BACILLUS THURINGIENSIS SUBSPECIES

The classification of Bacillus thuringiensis based on the serological analysis of the flagella antigens was introduced in the early 1960s (de Barjac & Bonnefoi, 1962). This classification by serotype has been supplemented by morphological and biochemical criteria (de Barjac, 1981). Clutill (1977), explains that only 13 Bacillus thuringiensis subspecies were toxic to lepidopteran Larva only. And apparently Nematode (Narva et; al., 1991) enlarged the host range and markedly increased the number of subspecies up to the end of 1998, over 67 subspecies based on flagella H – Serovars had been identified.

ECOLOGY AND PREVALENCE OF BACILLUS THURINGRENSIS

Although our knowledge about Bacillus thuringiensis occurs naturally and it can also be added to an ecosystem artificially to control pest, prevalence of

Bacillus thuringiensis in nature can be said as “natural” and can be isolated when there is no previous record of application of the organism for pest control.