Bacillus thuringiensis
Bacillus thuringiensis | |
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Spores and bipyramidal crystals of Bacillus thuringiensis morrisoni strain T08025 | |
Scientific classification | |
Domain: | Bacteria |
Phylum: | Bacillota |
Class: | Bacilli |
Order: | Bacillales |
Family: | Bacillaceae |
Genus: | Bacillus |
Species: | B. thuringiensis
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Binomial name | |
Bacillus thuringiensis Berliner 1915
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Subspecies | |
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Bacillus thuringiensis (or Bt) is a
During
As a toxic mechanism, cry proteins bind to specific receptors on the membranes of mid-gut (
Taxonomy and discovery
In 1902, B. thuringiensis was first discovered in
In 1976, Robert A. Zakharyan reported the presence of a plasmid in a strain of B. thuringiensis and suggested the plasmid's involvement in endospore and crystal formation.
Species group placement
B. thuringiensis is placed in the Bacillus cereus group which is variously defined as: seven closely related species: B. cereus sensu stricto (
Subspecies
There are several dozen recognized subspecies of B. thuringiensis. Subspecies commonly used as insecticides include B. thuringiensis subspecies kurstaki (Btk), subspecies israelensis (Bti) and subspecies aizawai (Bta).[19][20][21][22] Some Bti lineages are clonal.[18]
Genetics
Some strains are known to carry the same genes that produce
The proteins that B. thuringiensis is most known for are encoded by cry genes.[23] In most strains of B. thuringiensis, these genes are located on a plasmid (in other words cry is not a chromosomal gene in most strains).[24][25][26][18] If these plasmids are lost it becomes indistinguishable from B. cereus as B. thuringiensis has no other species characteristics. Plasmid exchange has been observed both naturally and experimentally both within B.t. and between B.t. and two congeners, B. cereus and B. mycoides.[18]
Various strains including
The genomes of the B. cereus group may contain two types of introns, dubbed group I and group II. B.t strains have variously 0–5 group Is and 0–13 group IIs.[18]
There is still insufficient information to determine whether chromosome-plasmid coevolution to enable adaptation to particular environmental niches has occurred or is even possible.[18]
Common with B. cereus but so far not found elsewhere – including in other members of the species group – are the
Proteome
It has a similar proteome diversity to its close relative B. cereus.[18]
Into the BT Cotton protein is 'Crystal protein'.
Mechanism of insecticidal action
Upon sporulation, B. thuringiensis forms crystals of two types of
Cry toxins have specific activities against insect species of the orders
A B. thuringiensis small RNA called BtsR1 can silence the Cry5Ba toxin expression when outside the host by binding to the RBS site of the Cry5Ba toxin transcript to avoid nematode behavioral defenses. The silencing results in an increase of the bacteria ingestion by C. elegans. The expression of BtsR1 is then reduced after ingestion, resulting in Cry5Ba toxin production and host death.[33]
In 1996 another class of insecticidal proteins in Bt was discovered: the vegetative insecticidal proteins (Vip; InterPro: IPR022180).[34][35] Vip proteins do not share sequence homology with Cry proteins, in general do not compete for the same receptors, and some kill different insects than do Cry proteins.[34]
In 2000, a novel subgroup of Cry protein, designated parasporin, was discovered from non-insecticidal B. thuringiensis isolates.
Use of spores and proteins in pest control
Spores and crystalline insecticidal proteins produced by B. thuringiensis have been used to control insect pests since the 1920s and are often applied as liquid sprays.[39] They are now 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, and are used in organic farming;[28] however, the manuals for these products do contain many environmental and human health warnings,[40][41] and a 2012 European regulatory peer review of five approved strains found, while data exist to support some claims of low toxicity to humans and the environment, the data are insufficient to justify many of these claims.[42]
New strains of Bt are developed and introduced over time[43] as insects develop resistance to Bt,[44] or the desire occurs to force mutations to modify organism characteristics[45][clarification needed], or to use homologous recombinant genetic engineering to improve crystal size and increase pesticidal activity,[46] or broaden the host range of Bt and obtain more effective formulations.[47] Each new strain is given a unique number and registered with the U.S. EPA[48] and allowances may be given for genetic modification depending on "its parental strains, the proposed pesticide use pattern, and the manner and extent to which the organism has been genetically modified".[49] Formulations of Bt that are approved for organic farming in the US are listed at the website of the Organic Materials Review Institute (OMRI)[50] and several university extension websites offer advice on how to use Bt spore or protein preparations in organic farming.[51][29]
Use of Bt genes in genetic engineering of plants for pest control
The Belgian company
Usage
In 1995, potato plants producing CRY 3A Bt toxin were approved safe by the
In 1996, genetically modified maize producing Bt Cry protein was approved, which killed the European corn borer and related species; subsequent Bt genes were introduced that killed corn rootworm larvae.[60]
The Bt genes engineered into crops and approved for release include, singly and stacked: Cry1A.105, CryIAb, CryIF, Cry2Ab,
Corn genetically modified to produce VIP was first approved in the US in 2010.[63]
In India, by 2014, more than seven million cotton farmers, occupying twenty-six million acres, had adopted Bt cotton.[64]
Monsanto developed a soybean expressing Cry1Ac and the glyphosate-resistance gene for the Brazilian market, which completed the Brazilian regulatory process in 2010.[65][66]
Bt aspen - specifically
Safety studies
The use of Bt toxins as plant-incorporated protectants prompted the need for extensive evaluation of their safety for use in foods and potential unintended impacts on the environment.[68]
Dietary risk assessment
Concerns over the safety of consumption of genetically modified plant materials that contain
Toxicology studies
Animal models have been used to assess human health risk from consumption of products containing Cry proteins. The United States Environmental Protection Agency recognizes mouse acute oral feeding studies where doses as high as 5,000 mg/kg body weight resulted in no observed adverse effects.[70] Research on other known toxic proteins suggests that toxicity occurs at much lower doses[clarification needed], further suggesting that Bt toxins are not toxic to mammals.[71] The results of toxicology studies are further strengthened by the lack of observed toxicity from decades of use of B. thuringiensis and its crystalline proteins as an insecticidal spray.[72]
Allergenicity studies
Introduction of a new protein raised concerns regarding the potential for allergic responses in sensitive individuals. Bioinformatic analysis of known allergens has indicated there is no concern of allergic reactions as a result of consumption of Bt toxins.[73] Additionally, skin prick testing using purified Bt protein resulted in no detectable production of toxin-specific IgE antibodies, even in atopic patients.[74]
Digestibility studies
Studies have been conducted to evaluate the fate of Bt toxins that are ingested in foods. Bt toxin proteins have been shown to digest within minutes of exposure to simulated gastric fluids.[75] The instability of the proteins in digestive fluids is an additional indication that Cry proteins are unlikely to be allergenic, since most known food allergens resist degradation and are ultimately absorbed in the small intestine.[76]
Ecological risk assessment
Ecological risk assessment aims to ensure there is no unintended impact on non-target organisms and no contamination of natural resources as a result of the use of a new substance, such as the use of Bt in genetically modified crops. The impact of Bt toxins on the environments where transgenic plants are grown has been evaluated to ensure no adverse effects outside of targeted crop pests.[citation needed]
Persistence in environment
Concerns over possible environmental impact from accumulation of Bt toxins from plant tissues, pollen dispersal, and direct secretion from roots have been investigated. Bt toxins may persist in soil for over 200 days, with
Impact on non-target organisms
The toxic nature of Bt proteins has an adverse impact on many major crop pests, but ecological risk assessments have been conducted to ensure safety of beneficial non-target organisms that may come into contact with the toxins. Widespread concerns over toxicity in non-target lepidopterans, such as the monarch butterfly, have been disproved through proper exposure characterization, where it was determined that non-target organisms are not exposed to high enough amounts of the Bt toxins to have an adverse effect on the population.[81] Soil-dwelling organisms, potentially exposed to Bt toxins through root exudates, are not impacted by the growth of Bt crops.[82]
Insect resistance
Multiple insects have developed a resistance to B. thuringiensis. In November 2009, Monsanto scientists found the pink bollworm had become resistant to the first-generation Bt cotton in parts of Gujarat, India - that generation expresses one Bt gene, Cry1Ac. This was the first instance of Bt resistance confirmed by Monsanto anywhere in the world.[83][84] Monsanto responded by introducing a second-generation cotton with multiple Bt proteins, which was rapidly adopted.[83] Bollworm resistance to first-generation Bt cotton was also identified in Australia, China, Spain, and the United States.[85] Additionally, resistance to Bt was documented in field population of diamondback moth in Hawaii, the continental US, and Asia.[86] Studies in the cabbage looper have suggested that a mutation in the membrane transporter ABCC2 can confer resistance to Bt Cry1Ac.[87]
Secondary pests
Several studies have documented surges in "sucking pests" (which are not affected by Bt toxins) within a few years of adoption of Bt cotton. In China, the main problem has been with
Similar problems have been reported in India, with both
Controversies
The controversies surrounding Bt use are among the many genetically modified food controversies more widely.[97]
Lepidopteran toxicity
The most publicised problem associated with Bt crops is the claim that pollen from Bt maize could kill the monarch butterfly.[98] The paper produced a public uproar and demonstrations against Bt maize; however by 2001 several follow-up studies coordinated by the USDA had asserted that "the most common types of Bt maize pollen are not toxic to monarch larvae in concentrations the insects would encounter in the fields."[99][100][101][102] Similarly, B. thuringiensis has been widely used for controlling Spodoptera littoralis larvae growth due to their detrimental pest activities in Africa and Southern Europe. However, S. littoralis showed resistance to many strains of B. thuriginesis and were only effectively controlled by a few strains.[103]
Wild maize genetic mixing
A study published in Nature in 2001 reported Bt-containing maize genes were found in maize in its center of origin, Oaxaca, Mexico.[104] Another Nature paper published in 2002 claimed that the previous paper's conclusion was the result of an artifact caused by an inverse polymerase chain reaction and that "the evidence available is not sufficient to justify the publication of the original paper."[105] A significant controversy happened over the paper and Nature's unprecedented notice.[106]
A subsequent large-scale study in 2005 failed to find any evidence of genetic mixing in Oaxaca.[107] A 2007 study found the "transgenic proteins expressed in maize were found in two (0.96%) of 208 samples from farmers' fields, located in two (8%) of 25 sampled communities." Mexico imports a substantial amount of maize from the U.S., and due to formal and informal seed networks among rural farmers, many potential routes are available for transgenic maize to enter into food and feed webs.[108] One study found small-scale (about 1%) introduction of transgenic sequences in sampled fields in Mexico; it did not find evidence for or against this introduced genetic material being inherited by the next generation of plants.[109][110] That study was immediately criticized, with the reviewer writing, "Genetically, any given plant should be either non-transgenic or transgenic, therefore for leaf tissue of a single transgenic plant, a GMO level close to 100% is expected. In their study, the authors chose to classify leaf samples as transgenic despite GMO levels of about 0.1%. We contend that results such as these are incorrectly interpreted as positive and are more likely to be indicative of contamination in the laboratory."[111]
Colony collapse disorder
As of 2007, a new phenomenon called colony collapse disorder (CCD) began affecting bee hives all over North America. Initial speculation on possible causes included new parasites, pesticide use,[112] and the use of Bt transgenic crops.[113] The Mid-Atlantic Apiculture Research and Extension Consortium found no evidence that pollen from Bt crops is adversely affecting bees.[99][114] According to the USDA, "Genetically modified (GM) crops, most commonly Bt corn, have been offered up as the cause of CCD. But there is no correlation between where GM crops are planted and the pattern of CCD incidents. Also, GM crops have been widely planted since the late 1990s, but CCD did not appear until 2006. In addition, CCD has been reported in countries that do not allow GM crops to be planted, such as Switzerland. German researchers have noted in one study a possible correlation between exposure to Bt pollen and compromised immunity to Nosema."[115] The actual cause of CCD was unknown in 2007, and scientists believe it may have multiple exacerbating causes.[116]
Beta-exotoxins
Some isolates of B. thuringiensis produce a class of insecticidal small molecules called beta-exotoxin, the common name for which is thuringiensin.[117] A consensus document produced by the OECD says: "Beta-exotoxins are known to be toxic to humans and almost all other forms of life and its presence is prohibited in B. thuringiensis microbial products".[118] Thuringiensins are nucleoside analogues. They inhibit RNA polymerase activity, a process common to all forms of life, in rats and bacteria alike.[119]
Other hosts
New nomenclature for pesticidal proteins (Bt toxins)
Bacillus thuringiensis is no longer the sole source of pesticidal proteins. The Bacterial Pesticidal Protein Resource Center (BPPRC) provides information on the rapidly expanding field of pesticidal proteins for academics, regulators, and research and development personnel.[120][121][122]
See also
- Biological insecticides
- Genetically modified food
- Western corn rootworm
- Cry1Ac
- Diamondback moth
- Sterile insect technique
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- de Maagd RA, Bravo A, Crickmore N (April 2001). "How Bacillus thuringiensis has evolved specific toxins to colonize the insect world". Trends in Genetics. 17 (4): 193–9. PMID 11275324.
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- Pigott CR, Ellar DJ (June 2007). "Role of receptors in Bacillus thuringiensis crystal toxin activity". Microbiology and Molecular Biology Reviews. 71 (2): 255–81. PMID 17554045.
- Tabashnik BE, Van Rensburg JB, Carrière Y (December 2009). "Field-evolved insect resistance to Bt crops: definition, theory, and data". Journal of Economic Entomology. 102 (6): 2011–25. S2CID 2325989.
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