by Piter Kehoma Boll
With the current raise in genetically modified crops and all controversies around them, you probably heard about things such as Bt corn and Bt cotton. But do you know what Bt means?
Bt stands for Bacillus thuringiensis, and this is going to be our fellow for today. A Gram-positive bacterium, Bt is found in many environments, including the soil, the surface of several plants and in the gut of several species of caterpillars. Belonging to the large and heterogeneous genus Bacillus, Bt is very closely related to, and sometimes considered as being of the same as, Bacillus cereus, which can can foodborne illness, and Bacillus anthracis, the species that causes anthrax. The main differences between these three species relies on their plasmids (small DNA pieces in a bacterium’s cytoplasm), while the genetic composition of their chromosome is basically the same.
As all species of Bacillus, Bt can sporulate, i.e., convert itself into a dormant form called endospore (sometimes wrongly named spore) when environmental conditions are not favorable. During sporulation, Bt forms crystals of delta-endotoxins, a proteinaceous inseticide also named crystal proteins or cry proteins. Cry proteins are encoded by cry genes, which are located in plasmids and not in the bacterial chromosome. When insects and nematodes ingest those crystals, they are denatured in the alkaline environment of the animal’s gut and become soluble. In soluble phase, the crystals are digested and liberate their toxins, which then paralyze the digestive tract and make the animal starve to death. The number of different cry proteins in Bt is really large, indicating a yet unknown selective pressure to the development of such a sophisticated defense mechanism.
Due to this strong insecticidal effect, Bt endospores and cry proteins have been used to control insect pests during the last century. The insecticide is usually applied as a spray and can be bought under different trademarks. However, because of natural selection, the pests end up developing resistance to the toxins and new strains of Bt are constantly produced to originate newer varieties of the insecticide.
Since the 1980s, there have been studies on the production of genetically modified organisms that incorporate Bt genes responsible for the production of cry proteins. Currently, the two most widely cultivated genetically modified crops containing Bt genes are Bt corn and Bt cotton. The cry genes of Bt have been introduced in the DNA of these species, allowing them to synthesize cry proteins. When an insect eats the leaves of such plants it ingests the crystals and dies as if it would have eaten the spores of Bt. And in fact, that is what happened. The introduction of Bt corn, for example, strongly reduced the attack of several corn pests, such as the corn borer.
Although the use of Genetically Modified Organisms (GMOs) is still seen as bad by many people, most studies have shown that they are relatively safe compared to many other human-interference activities. Bt corn and Bt cotton were shown to be safe for non-targeted organisms and to the environment as a whole. The main problem with GMOs is the fact that the technology to produce them lies in the hands of giant profit-maker companies.
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Helgason, E.; Økstand, O. A.; Caugant, D. A.; Johansen, H. A.; Fouet, A.; Mock, M.; Hegna, I.; Kolstø, A.-B. (2000) Bacillus anthracis, Bacillus cereus, and Bacillus thuringiensis — One species on the basis of genetic evidence. Applied and Environmental Microbiology, 66(6): 2627–2630. doi:
Schnepf, E.; Crickmore, N.; Van Rie, J.; Lereclus, D.; Baum, J.; Feitelson, J.; Zeigler, D. R.; Dean, D. H. (1998) Bacillus thuringiensis and its pesticidal crystal proteins. Microbiology and Molecular Biology Reviews, 62(3): 775–806.
Wikipedia. Bacillus thuringiensis. Available at < https://en.wikipedia.org/wiki/Bacillus_thuringiensis >. Access on December 28, 2018.
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