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      Bacillus thuringiensis Toxins: An Overview of Their Biocidal Activity

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          Abstract

          Bacillus thuringiensis (Bt) is a Gram positive, spore-forming bacterium that synthesizes parasporal crystalline inclusions containing Cry and Cyt proteins, some of which are toxic against a wide range of insect orders, nematodes and human-cancer cells. These toxins have been successfully used as bioinsecticides against caterpillars, beetles, and flies, including mosquitoes and blackflies. Bt also synthesizes insecticidal proteins during the vegetative growth phase, which are subsequently secreted into the growth medium. These proteins are commonly known as vegetative insecticidal proteins (Vips) and hold insecticidal activity against lepidopteran, coleopteran and some homopteran pests. A less well characterized secretory protein with no amino acid similarity to Vip proteins has shown insecticidal activity against coleopteran pests and is termed Sip (secreted insecticidal protein). Bin-like and ETX_MTX2-family proteins (Pfam PF03318), which share amino acid similarities with mosquitocidal binary (Bin) and Mtx2 toxins, respectively, from Lysinibacillus sphaericus, are also produced by some Bt strains. In addition, vast numbers of Bt isolates naturally present in the soil and the phylloplane also synthesize crystal proteins whose biological activity is still unknown. In this review, we provide an updated overview of the known active Bt toxins to date and discuss their activities.

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          Bacillus thuringiensis and its pesticidal crystal proteins.

          During the past decade the pesticidal bacterium Bacillus thuringiensis has been the subject of intensive research. These efforts have yielded considerable data about the complex relationships between the structure, mechanism of action, and genetics of the organism's pesticidal crystal proteins, and a coherent picture of these relationships is beginning to emerge. Other studies have focused on the ecological role of the B. thuringiensis crystal proteins, their performance in agricultural and other natural settings, and the evolution of resistance mechanisms in target pests. Armed with this knowledge base and with the tools of modern biotechnology, researchers are now reporting promising results in engineering more-useful toxins and formulations, in creating transgenic plants that express pesticidal activity, and in constructing integrated management strategies to insure that these products are utilized with maximum efficiency and benefit.
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            Bacillus thuringiensis: A story of a successful bioinsecticide.

            Bacillus thuringiensis (Bt) bacteria are insect pathogens that rely on insecticidal pore forming proteins known as Cry and Cyt toxins to kill their insect larval hosts. At least four different non-structurally related families of proteins form the Cry toxin group of toxins. The expression of certain Cry toxins in transgenic crops has contributed to an efficient control of insect pests resulting in a significant reduction in chemical insecticide use. The mode of action of the three domain Cry toxin family involves sequential interaction of these toxins with several insect midgut proteins facilitating the formation of a pre-pore oligomer structure and subsequent membrane insertion that leads to the killing of midgut insect cells by osmotic shock. In this manuscript we review recent progress in understanding the mode of action of this family of proteins in lepidopteran, dipteran and coleopteran insects. Interestingly, similar Cry-binding proteins have been identified in the three insect orders, as cadherin, aminopeptidase-N and alkaline phosphatase suggesting a conserved mode of action. Also, recent data on insect responses to Cry toxin attack is discussed. Finally, we review the different Bt based products, including transgenic crops, that are currently used in agriculture. Copyright © 2011 Elsevier Ltd. All rights reserved.
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              Biochemistry and genetics of insect resistance to Bacillus thuringiensis.

              Bacillus thuringiensis (Bt) is a valuable source of insecticidal proteins for use in conventional sprayable formulations and in transgenic crops, and it is the most promising alternative to synthetic insecticides. However, evolution of resistance in insect populations is a serious threat to this technology. So far, only one insect species has evolved significant levels of resistance in the field, but laboratory selection experiments have shown the high potential of other species to evolve resistance against Bt. We have reviewed the current knowledge on the biochemical mechanisms and genetics of resistance to Bt products and insecticidal crystal proteins. The understanding of the biochemical and genetic basis of resistance to Bt can help design appropriate management tactics to delay or reduce the evolution of resistance in insect populations.
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                Author and article information

                Contributors
                Role: External Editor
                Journal
                Toxins (Basel)
                Toxins (Basel)
                toxins
                Toxins
                MDPI
                2072-6651
                11 December 2014
                December 2014
                : 6
                : 12
                : 3296-3325
                Affiliations
                [1 ]Instituto de Agrobiotecnología, CSIC-UPNA-Gobierno de Navarra, Campus Arrosadía, Mutilva Baja, 31192 Navarra, Spain; E-Mail: leopoldo.palma@ 123456unavarra.es
                [2 ]Grupo de Protección Cultivos, Departamento de Producción Agraria, Escuela Técnica Superior de Ingenieros Agrónomos, Universidad Pública de Navarra, Pamplona, 31006 Navarra, Spain; E-Mails: dmunoz@ 123456unavarra.es (D.M.); jesus.murillo@ 123456unavarra.es (J.M.)
                [3 ]Cardiff School of Biosciences, Cardiff University, Park Place, Cardiff CF10 3AT, UK; E-Mail: Berry@ 123456cf.ac.uk
                Author notes
                [* ]Author to whom correspondence should be addressed; E-Mail: pcm92@ 123456unavarra.es ; Tel.: +34-948-168-004; Fax: +34-948-169-300.
                Article
                toxins-06-03296
                10.3390/toxins6123296
                4280536
                25514092
                21500960-33f5-4849-ae83-12f991fbe588
                © 2014 by the authors; licensee MDPI, Basel, Switzerland.

                This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 01 September 2014
                : 07 November 2014
                : 03 December 2014
                Categories
                Review

                Molecular medicine
                bacillus thuringiensis,bt biopesticides,toxic activity,cry toxins,cyt toxins,vip toxins,sip toxins,parasporins

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