Profile of a bacterium
It has been known for over a hundred years that certain common, soil-dwelling bacteria - Bacillus thuringiensis (Bt) - have a toxic and deadly effect on insects. This is due to the Bt toxin, a protein produced by the bacteria. This feature of the bacterium was harnessed as a means of crop protection back in the middle of the last century, when the first Bacillus thuringiensis preparations were brought onto the market as bio-insecticides. Today a broad spectrum of these Bt preparations is used, especially in organic farming. Since the 1980s, genetic engineers have also shown an interest in the bacterium. When the gene for the bacterial protein is transferred to plants, the plants themselves produce the Bt toxin, which protects them against chewing pests.
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The bacterium was first discovered in silkworms in 1901 by a Japanese scientist, who named it Bacillus sotto, although he remained unaware of its special characteristics. Ten years later, the German scientist Ernst Berliner isolated the bacterium from diseased flour moths and assumed it was responsible for the sudden-collapse disease (“sotto” or “flacherie”), which was affecting the insects. Since he had obtained the first sample from a mill in Thuringia, he called the bacterium Bacillus thuringiensis. Field trials with Bacillus thuringiensis to control the European corn borer were being conducted as early as the late 1920s and in 1938 the first commercial Bt preparation (Sporeine) came onto the market in France. However, it was not until the sixties that its use became widespread. In 1964 Biospor became the first Bt preparation to be licensed as a pesticide in Germany. |
Since the introduction of the first Bt preparations in crop protection, more and more new, previously unknown strains of Bacillus thuringiensis have been identified, each of which affects only certain insect groups. With the discovery in 1970 of the particularly virulent Bt strain B. thuringiensis kurstaki, which is effective against the larvae of certain butterflies and moths, and the discovery in 1983 of the B. thuringiensis tenebrionis strain, which is effective against certain beetles, including the Colorado potato beetle, the range of Bt agents available has grown considerably.
Targeted use
Not all Bt strains are toxic to insects, but of the toxic strains, around 170 Bt toxins have now been identified. Their spectrum of activity is restricted to three insect orders: butterflies and moths, leaf beetles and two-winged flies and midges.
In 1989 the toxins were divided into five main classes according to their spectrum of activity, uniformity of gene sequences and molecular size (cry =crystal):
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CryI: acts specifically against butterflies and moths
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CryII: butterflies, moths and two-winged flies
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CryIII: beetles
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CryIV: two-winged flies
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Cry V: beetles, butterflies and moths
The fact that Bt proteins are so specific is a key advantage of Bt preparations. The individual Bt protein variants target the pests of a specific crop without harming other species considered to be beneficial. In addition, the Bt protein is harmless to mammals and humans.
Commercially available Bt preparations consist of dry bacterial spores and the crystalline toxin. Today they are used mainly in viticulture, forestry, green spaces and in potato, fruit and vegetable cultivation. They are particularly important in organic farming; Bt preparations account for around 90% of all bio-insecticides. By contrast, they comprise only one percent of agro-chemicals.
How the Bt toxin works
The Bt bacteria initially produce the active protein in a non-toxic form (protoxin) as a crystalline protein. This protein is converted to a toxic variant only in the intestine of certain chewing pests. The crystalline protein is absorbed by the insect with food. It dissolves in the intestinal tract, where it is activated by special enzymes in the intestinal juice, known as proteases, and finally binds to specific receptors in the intestinal wall. It integrates with the intestinal wall to create pores. This causes the intestinal wall to become perforated, leading to the insect's death.
In September 2006 scientists from the University of Wisconsin-Madison reported in the scientific journal PNAS, Proceedings of the National Academy of Sciences that Bt protein cannot become active without the help of gut bacteria. They fed insects first with food containing antibiotics, to kill off the gut bacteria. When the insects were subsequently given food containing the Bt protein, no insecticidal effect was observed. It is assumed that the insect's death is caused by blood poisoning as result of the bacteria passing through the pores into the blood stream.
Bt toxin in transgenic plants: Different to bacterial toxin?
Using genetic engineering, the Bt protein genes isolated from Bacillus thuringiensis can be transferred to plants. As with all proteins, the "blueprint" for the Bt protein is encoded in a specific DNA sequence (gene). If the Bt protein gene isolated from bacteria is inserted into the DNA of a plant, the plant itself produces Bt toxin. In 1995 the first Bt plant, maize, was approved in the USA. Today Bt maize is grown on around 14 million hectares, mainly in the USA.
The active toxins from Bt plants and from pesticides containing the active substance Bt have a similar mode of action. The Bt protein is present as a protoxin in both. Only in the alkaline intestinal environment of sensitive insects is the protoxin, or toxin precursor, converted to an active toxin by specific enzymes (proteases).
The difference is that the Bt genes introduced into the plants have been shortened and adapted to the plants. The protein in the plants is present not in crystal form, but as a solution. Extensive studies from several safety research projects have so far failed to confirm speculations that insects other than the "target organism" could potentially be harmed by the toxin as result.
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