Bacillus thuringiensis

All you want to know about Bacillus thuringiensis

Bacillus thuringiensis
Spores and bipyramidal crystals of Bacillus thuringiensis morrisoni strain T08025
Spores and bipyramidal crystals of Bacillus thuringiensis morrisoni strain T08025
Scientific classification
Kingdom: Eubacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillales
Family: Bacillaceae
Genus: Bacillus
Species: thuringiensis
Binomial name
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 gut of caterpillars of various types of moths and butterflies, as well as on the dark surface of plants.[1]

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.[1]

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. 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. [2]

Contents

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.[3][4]

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.[5]

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

Bt-toxins present in peanut leaves (bottom image) protect it from extensive damage caused by European corn borer larvae (top image).[6]

Usage

Bt crops (in corn and cotton) were planted on 281,500 km² in 2006 (165,600 km² of Bt corn and 115900 km² of Bt cotton). This was equivalent to 11.1% and 33.6% respectively of global plantings of corn and cotton in 2006.[7] 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" .[8]

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)[9] 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.[7]

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, and
iv) replaces the use of synthetic pesticides in the environment. The latter observation has been well documented world-wide.[7]

Safety

An independent study commissioned by the Austrian Ministry of Health has found significant infertility effects from mice fed genetically modified (GM) corn. Monsanto, which has a near-world monopoly on all agricultural seeds tried to, not only stop the study from the beginning, but then tried to block the results from being published.

In one study design where mice were continuously breeding. Mice fed on GM corn had no litters or produced less offspring after the third or fourth litters, than those fed on conventional corn. The differences were statistically significant.

The corn, sold by Monsanto, contains a gene that produces the toxic “Bt” pesticide in every cell and in every bite. The results raise the question whether this toxin (or some other unpredictable change in the GM corn) might be contributing to the rise in infertility, allergies or other immune disorders in North America.

http://bmgfj.cms.apa.at/cms/site/attachments/3/2/9/CH0810/CMS1226492832306/forschungsbericht_3-2008_letztfassung.pdf

A recent study funded by the European arm of Greenpeace, suggested the possibility of a slight but statistically meaningful risk of liver damage in rats.[10] 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.[11]

Limitations to Bt crops

Kenyans examining insect-resistant transgenic Bt corn.

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.[12] The same researcher has now reported the first documented case of pest resistance to biotech cotton.[13]

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.[14] 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 stragegy. 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.[15] 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.[16]

Possible problems

The most celebrated problem ever associated with Bt crops was the claim that pollen from Bt maize could kill the monarch butterfly.[17] This report was puzzling because the pollen from most maize hybrids contains much lower levels of Bt than the rest of the plant[18] 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.[19] The weight of the evidence is that Bt crops do not pose a risk to the monarch butterfly.[20]

There was also a report in Nature, that Bt maize was contaminating maize in its center of origin.[21] Nature later "concluded that the evidence available is not sufficient to justify the publication of the original paper."[22] A subsequent large-scale study failed to find any evidence of contamination in Oaxaca.[23]

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[24] to the use of Bt resistant transgenic crops.[25] 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. CCD has since been attributed to a new virus, unrelated to Bt crops.[26]

References

  1. ^ a b Madigan, Michael; Martinko, John (editors) (2005). Brock Biology of Microorganisms, 11th ed., Prentice Hall. ISBN 0-13-144329-1. 
  2. ^ [1]DNA shuffling of Cry proteins
  3. ^ Höfte H, de Greve H, Seurinck J, Jansens S, Mahillon J, Ampe C, Vandekerckhove J, Vanderbruggen H, van Montagu M, Zabeau M (1986). "Structural and functional analysis of a cloned delta endotoxin of Bacillus thuringiensis berliner 1715". Eur J Biochem 161 (2): 273–80. doi:10.1111/j.1432-1033.1986.tb10443.x. PMID 3023091. 
  4. ^ Vaeck M, Reynaerts A, Hofte A, Jansens S, De Beuckeleer M, Dean C, Zabeau M, Van Montagu M, Leemans J (1987). "Transgenic plants protected from insect attack". Nature 328: 33–37. doi:10.1038/328033a0, http://www.nature.com/nature/journal/v328/n6125/abs/328033a0.html. 
  5. ^ Broderick N, Raffa K, Handelsman J (2006). "Midgut bacteria required for Bacillus thuringiensis insecticidal activity". Proc Natl Acad Sci U S a 103 (41): 15196–9. doi:10.1073/pnas.0604865103. PMID 17005725. 
  6. ^ Jan Suszkiw (November 1999.). "Tifton, Georgia: A Peanut Pest Showdown". Agricultural Research magazine. Retrieved on 2007-05-23.
  7. ^ a b c Microsoft Word - global_impactstudy_2006_v1_finalPGEconomics.doc
  8. ^ Smale M, Zambrano P, Cartel M (2006). "Bales and Balance: A Review of the Methods Used to Assess the Economic Impact of Bt Cotton on Farmers in Developing Economies". AgBioForum 9 (3): 195-212. 
  9. ^ A Method to Measure the Environmental Impact of Pesticides
  10. ^ Séralini, et al: New analysis of a rat feeding study with a genetically modified maize reveals signs of hepatorenal toxicty, Archives of Environmental Contamination and Toxicology, Springer Science, Published online 13 March 2007.
  11. ^ http://www.efsa.eu.int/EFSA/News_PR/PR_MON863__final,0.pdf
  12. ^ "Organic Mystery," Scientific American, December, 2006, p. 33, quote by Bruce Tabashnik of the University of Arizona. [2]
  13. ^ First documented case of pest resistance to biotech cotton
  14. ^ Roush, R.T. 1997. Bt-transgenic crops: just another pretty insecticide or a chance for a new start in resistance management? Pestic. Sci. 51:328-334.
  15. ^ VDong, H. Z. and Li, W. J. (2007) Variability of Endotoxin Expression in Bt Transgenic Cotton. Journal of Agronomy & Crop Science; 193:21-29.
  16. ^ Tabashnik, B.E., Y. Carriere, T.J. Dennehy, S. Morin, M.S. Sisterson, R.T. Roush, A.M. Shelton, and J.Z. Zhao. 2003. Insect resistance to transgenic Bt crops: Lessons from the laboratory and field. J. Econ. Entomol. 96:1031-1038.
  17. ^ Losey, J.E., L.S. Raynor, and M.E. Carter. 1999. Transgenic pollen harms monarch larvae. Nature 399:214
  18. ^ Mendelsohn, M., J. Kough, Z. Vaituzis, and K. Matthews. 2003. Are Bt crops safe? Nature Biotechnology 21:1003-1009
  19. ^ Hellmich, R.L., B.D. Siegfried, M.K. Sears, D.E. Stanley-Horr, M.J. Daniels, H.R. Mattila, T. Spencer, K.G. Bidne and L.C. Lewis. 2001. Monarch larvae sensitivity to Bacillus thuringiensis -purified proteins and pollen. Proceedings of the National Academy of Sciences USA 98:11925-11930
  20. ^ Bt Corn and Monarch Butterflies
  21. ^ Quist D and Chapela IH. 2001. Transgenic DNA introgressed into traditional maize landraces in Oaxaca, Mexico. Nature 414: 541-543
  22. ^ Editor, Nature. 2002. Editorial note. Nature 416: 601
  23. ^ S. Ortiz-García,* E. Ezcurra,*† B. Schoel,‡ F. Acevedo,§ J. Soberón,§¶ and A. A. Snow. Absence of detectable transgenes in local landraces of maize in Oaxaca, Mexico (2003–2004) 2005. Proc Natl Acad Sci U S A. 102(35): 12338–12343.
  24. ^ ARS: Questions and Answers: Colony Collapse Disorder
  25. ^ Latsch, Gunther. Are GM Crops Killing Bees?. Spiegel International. March 22, 2007. [3]
  26. ^ BBC NEWS | Science/Nature | Virus implicated in bee decline

See also

External links

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