|
|
|
|
Bacillus thuringiensis
|
| |
|
| |
Bacillus thuringiensis is a Gram-positive, soil-dwelling bacterium of the genus Bacillus. Additionally, B. thuringiensis also occurs naturally in the gut of caterpillars of various types of moths and butterflies, as well as on the dark 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.
Discussion
Ask a question about 'Bacillus thuringiensis' Start a new discussion about 'Bacillus thuringiensis' Answer questions from other users |
Encyclopedia
Bacillus thuringiensis is a Gram-positive, soil-dwelling bacterium of the genus Bacillus. Additionally, B. thuringiensis also occurs naturally in the gut of caterpillars of various types of moths and butterflies, as well as on the dark 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.
Zakharyan R.A et al. (1976,1977,1979) first reported the presence of plasmids in B. thuringiensis and suggested involvement of the plasmids in endospore/crystal formation. They also described the presence of large plasmid in the Cry+ variant of B. thuringiensis
Upon sporulation, B. thuringiensis forms crystals of proteinaceous insecticidal d-endotoxins (Cry toxins) which are encoded by cry genes,. It was determined that the "cry" genes are harbored in the plasmids in most strains of B. thuringiensis 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. When insects ingest toxin crystals the alkaline pH of their digestive tract causes the toxin to become activated. It becomes inserted into the insect's gut cell membranes forming a pore resulting in swelling, cell lysis and eventually killing the insect.
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.
Genetic engineering for pest control
Usage
Bt crops (in corn and cotton) were planted on 281,500 kmē in 2006 (165,600 kmē of Bt corn and 115900 kmē of Bt cotton). This was equivalent to 11.1% and 33.6% respectively of global plantings of corn and cotton in 2006. Claims of major benefits to farmers, including poor farmers in developing countries, have been made by advocates of the technology, and have been challenged by opponents. The task of isolating impacts of the technology is complicated by the prevalence of biased observers, and by the rarity of controlled comparisons (such as identical seeds, differing only in the presence or absence of the Bt trait, being grown in identical situations). The main Bt crop being grown by small farmers in developing countries is cotton, and a recent exhaustive review of findings on Bt cotton by respected and unbiased agricultural economists concluded that "the overall balance sheet, though promising, is mixed. Economic returns are highly variable over years, farm type, and geographical location" .
Environmental impacts appear to be positive during the first ten years of Bt crop use (1996-2005). One study concluded that insecticide use on cotton and corn during this period fell by 35.6 million kg of insecticide active ingredient which is roughly equal to the amount of pesticide applied to arable crops in the EU in one year. Using the Environmental Impact Quotient (EIQ) measure of the impact of pesticide use on the environment, the adoption of Bt technology over this ten year period resulted in 24.3% and 4.6% reduction respectively in the environmental impact associated with insecticide use on the cotton and corn area using the technology.
Advantages
There are several advantages in expressing Bt toxins in transgenic Bt crops:
- The level of toxin expression can be very high thus delivering sufficient dosage to the pest.
- The toxin expression is contained within the plant system and hence only those insects that feed on the crop perish.
- The toxin expression can be modulated by using tissue-specific promoters, and replaces the use of synthetic pesticides in the environment. The latter observation has been well documented world-wide.
Safety
Overall, Bt-modified crops appear to be safe for farmers and consumers. Additionally, the proteins produced by Bt have also been used in sprays in farming techniques for many years with seemingly no ill effects on environment or human health. Thus, Bt toxins are considered environmentally friendly by many farmers and may be a potential alternative to broad spectrum insecticides. The toxicity of each Bt type is limited to one or two insect orders, and is nontoxic to vertebrates and many beneficial arthropods. The reason is that Bt works by binding to the appropriate receptor on the surface of midgut epithelial cells. Any organism that lacks the appropriate receptors in its gut cannot be affected by Bt.
There is clear evidence from laboratory settings that Bt toxins can affect non-target organisms. Usually, but not always, affected organisms are closely related to intended targets (reviewed in Lovei and Arpaia 2005 and Hilbeck and Schmidt 2006). Typically, exposure is through the consumption of plant parts such as pollen or plant debris or through Bt ingested by their predatory food choices. Nevertheless, due to significant data gaps, the real-world consequences of Bt transgenics remains unclear.
Not all scientific reports on Bt safety have been positive. A 2007 study funded by the European arm of Greenpeace, suggested the possibility of a slight but statistically meaningful risk of liver damage in rats. While small statistically significant changes may have been observed, statistical differences are both probable and predictable in animal studies of this kind, and are known as Type I errors- that is, the probability of finding a false-positive due to chance alone. In this case, the number of positive results was within the statistically predicted range for Type I errors. The observed changes have been found to be of no biological significance by the European Food Safety Authority. A 2008 Austrian study investigating the usefulness of a long-term reproduction mouse model for GM crop safety reported that Bt-treated corn consumption in mice appeared to be correlated with reduced fertility via an unknown biochemical mechanism.
Limitations to Bt crops
Constant exposure to a toxin creates evolutionary pressure for pests resistant to that toxin. Already, a Diamondback moth population is known to have acquired resistance to Bt in spray form (i.e., not engineered) when used in organic agriculture. The same researcher has now reported the first documented case of pest resistance to biotech cotton.
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. To reduce the chance that an insect would become resistant to a Bt crop, the commercialization of transgenic cotton and maize in 1996 was accompanied with a management strategy to prevent insects from becoming resistant to Bt crops, and insect resistance management plans are mandatory for Bt crops planted in the USA and other countries. 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. This means that with sufficiently high levels of transgene expression, nearly all of the heterozygotes (S/s), the largest segment of the pest population carrying a resistance allele, will be killed before they reach maturity, thus preventing transmission of the resistance gene to their progenies. The planting of refuges (i. e., fields of non-transgenic plants) adjacent to fields of transgenic plants increases the likelihood that homozygous resistant (s/s) individuals and any surviving heterozygotes will mate with susceptible (S/S) individuals from the refuge, instead of with other individuals carrying the resistance allele. As a result, the resistance gene frequency in the population would remain low.
Nevertheless, there are limitations that can affect the success of the high-dose/refuge strategy. For example, 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. So, while the high-dose/refuge strategy has been successful at prolonging the durability of Bt crops, this success has also had much to do with key factors independent of management strategy, including low initial resistance allele frequencies, fitness costs associated with resistance, and the abundance of non-Bt host plants that have supplemented the refuges planted as part of the resistance management strategy.
Possible problems
The most celebrated problem ever associated with Bt crops was the claim that pollen from Bt maize could kill the monarch butterfly. This report was puzzling because the pollen from most maize hybrids contains much lower levels of Bt than the rest of the plant and led to multiple follow-up studies. In the end, it appears that the initial study was flawed; based on the way the pollen was collected, they collected and fed non-toxic pollen that was mixed with anther walls that did contain Bt toxin. The weight of the evidence is that Bt crops do not pose a risk to the monarch butterfly.
There was also a report in Nature that Bt maize was contaminating maize in its center of origin. Nature later "concluded that the evidence available is not sufficient to justify the publication of the original paper." A subsequent large-scale study, in 2005, failed to find any evidence of contamination in Oaxaca. However, further researchs confirmed initial findings concerning contamination of natural maize by transgenic maize .
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. However, there is no evidence of crossbreeding between maize and wild grasses.
As of 2007, a new phenomenon called Colony Collapse Disorder (CCD) is affecting bee hives all over North America. Initial speculation on possible causes ranged from cell phone and pesticide use to the use of Bt resistant transgenic crops. The 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. The actual cause of CCD remains unknown, scientists believe that it may have multiple causes.
See also
External links
- (National Pesticide Information Center)
- (National Pesticide Information Center)
- Research projects and results
-
The European Corn Borer is an economically significant pest of corn in the USA. This pest damages the stalks of corn plants in its second generation.
- A variant of Bt toxin, known as Kurstaki HD-1, has insecticidal effects on the Corn Borer. The gene for this variant, CrylA(b), has been transformed into corn plants creating a resistant GM strain of corn. This strain has been crossed with commercial corn strains to create economical, resistant strains.
- The levels of Bt toxin in these strains is low, but enough to kill the pest. The toxin causes no harm to other organisms or the crop.
-
- Research was conducted to find the effect of this Gm Bt Crop on monarch butterflies which feed on milk-weed which grows at the edge of corn fields. The research included an experiment in which the butterflies were given milkweed dusted with the toxin as a food source. The experiment showed that the toxin had strong effects on the butterflies. This result was published in Nature as well as in many newspapers and there was public outrage at the risk presented to the endangered and endearing insects.
-
- The scientific community took a different approach. As soon as the research was published it was immediately criticized. The experiment in which milkweed was dusted with Bt toxin did not reflect the real doses of Bt which would be received by milkweed bordering GM crops. Also, it was shown that Monarch butterflies, when given a choice, would not choose to eat dusted plants. The results of the research were debunked. The popular press took no note of the findings against the first research. This is one reason why public opinion of Bt GM crops and GM crops in general is so low.
|
| |
|
|
