Bacillus thuringiensis
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| Bacillus thuringiensis Berliner 1915 |
Bacillus thuringiensis is a Gram-positive, soil dwelling bacterium of the genus Bacillus. Additionally, B. thuringiensis also occurs naturally in the caterpillars of some moths and butterflies, as well as on the surface of plants.[1]
B. thuringiensis was discovered 1901 in Japan and 1911 in Germany by Ernst Berliner, who discovered a disease called Schlaffsucht in flour moth caterpillars. B. thuringiensis is closely related to B. cereus, a soil bacterium, and B. anthracis, the cause of anthrax: the three organisms only differ in their plasmids. Like other members of the genus, all three are aerobes capable of producing endospores.[1]
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[edit] Use in pest control
Spores of B. thuringiensis, as well as proteins created by the organism are used as Lepidopteran-specific insecticides under trade names such as Dipel and Thuricide. Because of their specificity, these pesticides are regarded as Environmentally friendly, with little or no effect on humans, wildlife, pollinators, and most other beneficial insects. The Belgian company Plant Genetic Systems was the first company (in 1985) to develop genetically engineered (tobacco) plants with insect tolerance by expressing genes encoding for insecticidal proteins from Bacillus thuringiensis (Bt) [2][3].
The material is distributed, usually in a liquid spray on the leaves of affected plants, where the pesticide must be eaten to be effective. Previously it was thought that the protein crystals break down cells within the gut of the caterpillars. Recent research has disproven this theory, as the midgut bacteria in the insects are required for the insecticidal activity (http://www.pnas.org/cgi/reprint/0604865103), and the insects die sooner when intoxicated than when starved. The exact mechanism of the protein crystal toxicity is still unknown.
Bacillus thuringiensis var israelensis, a strain of B. thuringiensis is widely used as a larvicide against mosquito larvae, where it is also considered an environmentally friendly technique of mosquito control.
[edit] Genetic engineering for pest control
Bacillus thuringiensis, or Bt, is an endospore forming, soil-dwelling bacterium. The bacteria forms protein crystal δ-endotoxins from Cry genes. These toxins have effects on Lepidoptera (Caterpillars) and Coleoptera (Beetles) species. These toxins are used as biological control in organic farming and as transgenes in GM crops.
[edit] Usage
In 2000 more than 115,000 square kilometres of Bt transgenic crops were grown, constituting 19% of the worlds GM crops. There is potential for Bt GM crops to take up 33% of the insecticide market. The current use of transgenic Bt crops reduces the number of chemical insecticide treatments by more than 7.7 million acres (31,000 km²) per year.
[edit] Advantages
Transgenic Bt crops have even distribution of the toxin throughout the plant. The treatment is constant unlike chemical spraying which creates many pauses.
[edit] Safety
Transgenic crops, including Bt crops, are safe for the farmers and for consumers. The toxin is insect specific and poses no danger to humans or other vertebrates.
[edit] Problems
The expression of the Bt gene can vary. For instance, if the temperature is not ideal this stress can lower the toxin production and make the plant more susceptible. Secondary pests are not controlled by Bt transgenic crops. Due to the constant exposure to the toxin an evolutionary selective pressure is created for resistant pests. There is also a hypothetical risk that for example, transgenic maize will crossbreed with wild grass variants, and that the Bt-gen will end up in a natural environment, retaining its toxicity.
Reasons for sudden dieing of bt cotton plants in the main field is unknown till now to the bt cotton developers across the world.
[edit] Fighting Resistance
Non-Bt-Gm crop refuges could be created to allow some non-resistant insects to survive and maintain a susceptible population. Moderate expression of the transgene would also achieve the same end. Creating a mosaic GM crop expressing many different Bt toxins would have a greater chance of eliminating the entire pest population. [4] Another approach is to provide "refuges" of non-Bt crops, which is often required by legislation. The aim is to encourage a large population of pests so that any genes for resistance are greatly diluted. This appears to be successful.[5]
[edit] References
- ^ a b Madigan, Michael; Martinko, John (editors) (2005). Brock Biology of Microorganisms, 11th ed., Prentice Hall. ISBN 0-13-144329-1.
- ^ Hofte H, de Greve H, Seurinck J, Jansens S, Mahillon J, Ampe C, Vandekerckhove J, Vanderbruggen H, van Montagu M, Zabeau M, et al., Structural and functional analysis of a cloned delta endotoxin of Bacillus thuringiensis berliner 1715, Eur J Biochem. 1986 Dec 1;161(2):273-80.
- ^ Vaeck, M., A. Reynaerts, H. Hofte, S. Jansens, M. De Beuckeleer, C. Dean, M. Zabeau, M. Van Montagu & J. Leemans. 1987, Transgenic plants protected from insect attack, Nature 328: 33-37
- ^ Atkinson, H. Lecture: Engineering Resistance to Insects.
- ^ Bt Cotton Script - Australian Broadcasting Corporation's Science Show See also Controlling pests in cotton crops
[edit] External links
- Breakdown of the Bt toxin and effects on the environment Research projects and results
