Bacillus thuringiensis

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.

B. thuringiensis was discovered 1901 in Japan by Ishiwata 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 differ mainly in their plasmids. Like other members of the genus, all three are aerobes capable of producing endospores.

Upon sporulation, B. thuringiensis forms crystals of proteinaceous insecticidal δ-endotoxins (Cry toxins: Bacillus thuringiensis Toxin Nomenclature) which are encoded by cry genes. Cry toxins have specific activities against species of the orders Lepidoptera (Moths and Butterflies), Diptera (Flies and Mosquitoes) and Coleoptera (Beetles). Thus, B. thuringiensis serves as an important reservoir of Cry toxins and cry genes for production of biological insecticides and insect-resistant genetically modified crops.

Use in pest control
Spores and crystalline insecticidal proteins produced by B. thuringiensis are used as 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 cry genes from B. thuringiensis.

B. thurigiensis-based insecticides are often applied as liquid sprays on crop plants, where the insecticide must be ingested to be effective. It is thought that the solubilized toxins form pores in the midgut epithelium of susceptible larvae. Recent research has suggested that the midgut bacteria of susceptible larvae are required for B. thuringiensis insecticidal activity.

Bacillus thuringiensis serovar israelensis, a strain of B. thuringiensis is widely used as a larvicide against mosquito larvae, where it is also considered an environmentally friendly method of mosquito control.

Usage
In 2000 more than 115,000 square kilometers 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.

Advantages
There are several advantages in expressing Bt toxins in transgenic Bt crops: i). The level of toxin expression can be very high thus delivering sufficient dosage to the pest, ii). The toxin expression is contained within the plant system and hence only those insects that feed on the crop perish, iii). The toxin expression can be modulated by using tissue-specific promoters, iv). The resistance is inherited in a stable and Mendelian fashion and v). The toxin gene can be integrated in the chloroplast genome and thus the possibility of gene transfer via pollen is eliminated.

Safety
Bt crops appear to be safe for the farmers and for consumers. The toxin is insect specific and poses no known danger to humans, although the technology is too new for long-term studies to be available. Bt toxin is not inactivated by sun (as normal) and is present for a long time in the soil (see documentary). A recent study funded by the European arm of Greenpeace, while inconclusive, has shown the possibility of a slight but statistically meaningful risk of liver damage in rats.

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. More importantly, reduced late-season expression of toxin has been documented, possibly resulting from DNA methylation of the promoter.

Due to the constant exposure to the toxin an evolutionary pressure is created for resistant pests. Already, the Diamondback moth population is known to have adapted so that it now has a resistance to Bt in spray form. There is also a hypothetical risk that for example, transgenic maize will crossbreed with wild grass variants, and that the Bt-gene will end up in a natural environment, retaining its toxicity. An event like this would have ecological implications, as well as increasing the risk of Bt resistance arising in the general herbivore population.

As of 2007, a new phenomenon called Colony Collapse Disorder (CCD) is affecting bee hives all over North America. While its causes are still unknown, its link to the use of Bt resistant transgenic crops is causing concern among scientists. A research group called Mid-Atlantic Apiculture Research and Extension Consortium published a report on 2007-03-27 that found no evidence that pollen from Bt crops is adversely affecting bees. However, the study only investigated short term exposure. Studies on long term exposure still need to be investigated to determine if it causes changes in bee behaviour.

Fighting resistance
One method of reducing resistance is the creation of Non-Bt crop refuges to allow some non-resistant insects to survive and maintain a susceptible population. The refuge approach is required by legislation in some regions including the US and Europe. The aim is to encourage a large population of pests so that any genes for resistance are greatly diluted. This technique is based on the assumption that resistance genes will be recessive. It appears so far to be a successful method of delaying widespread resistance to Bt toxins. Alternately, creating a mosaic GM crop expressing many different Bt toxins would have a greater chance of eliminating the entire pest population and thus eliminating resistance alleles. To date, no planned extinction of an insect pest has been successful.