Bacillus thuringiensis

Bacillus thuringiensis is a bacterium that can be found in particular in the ground, but also to plant and insect cadavers. The produced by the bacterium Bt toxins are used for biological pest control in agriculture and forestry and in the control of disease-carrying mosquitoes.

Description

Bacillus thuringiensis as other species of the genus Bacillus, and rod-shaped Gram- positive. He is resting stages in the form of spores. It was described in 1901 in Japan by Ishiwatari Shigetane (Jap.石 渡 繁 胤) as Bacillus sotto and 1911 by Ernst Berliner ( 1880-1957 ) in Germany, who gave him the name Bacillus thuringiensis. Ishiwatari Shigetane found the bacillus in silkworms, Ernst Berliner discovered him in flour moth caterpillars from a mill in Thuringia. He also realized that this bacterium triggers the so-called flacherie in flour moth larvae. The various subspecies of B. thuringiensis produce over 200 different so-called Bt toxins, which act specifically lethal in certain insects. B. thuringiensis lives in company with plant roots. It is believed that the toxins of the bacteria retain the roots from damage by insects.

B. thuringiensis is closely related to Bacillus cereus and differs from it only bacterium - like Bacillus anthracis - by so-called plasmids. Like all other Bacillus species are aerobic and form endospores.

Bt toxin

B. thuringiensis produces crystalline proteins (Bt toxins, see figure), which act specifically toxic to different insect species of the orders beetles, butterflies, Hymenoptera and Diptera, and nematodes in plants, vertebrates, and humans, however, are ineffective. They are completely biodegradable.

The genetic information for the Bt toxins are located on plasmids. Depending on the strain expressing these one or more different so-called Cry and Cyt proteins that differ in their toxicity towards the different insect orders. There are more than 200 different Cry proteins. These toxic crystal proteins must first be dissolved in the insect gut and activated by a specific enzymatic cleavage of subunits before they can take effect. The amount and type of proteins varies here from tribe to tribe. Cyt and Cry proteins attach themselves to the intestinal cells, and create pores in the cell membrane, so that the intestinal cell is destroyed. For this to happen, the respective toxins must find specific attachment sites on the cell membrane, which explains its specific effect against certain insects.

Applications

Bt toxins are used in agriculture and forestry as pesticides and mosquito control.

Agriculture

In agriculture, Bt toxins have been used since 1938, but have no great significance in this form attained. In contrast, since their initial registration in 1996 grew the acreage of Bt plants rapidly.

Bt suspensions

Especially in organic farming Bt toxins are used in the form of suspensions against free -eating caterpillars. This agent is used in France since 1938 in the USA since 1950. Today different Bt toxins used against various insects.

B. thuringiensis israelensis is used in mosquito control. The mosquitoes are fought in the larval stage in the waters. The effects on other insect larvae, such as midges are small.

A genetic variant of this subspecies has been patented in the United States. Other subspecies are used B. thuringiensis kurstaki, B. thuringiensis aizawa, B. thuringiensis san diego, and B. thuringiensis Tenebrionis. As with other pesticides, the target organisms can develop resistance after a more or less long period of use. Compared to Bt toxins, this was first observed in the late 1980s in the diamondback moth.

Bt plants

→ Main article: Green genetic engineering, transgenic cotton, transgenic maize

From Bacillus thuringiensis transferred into crop plants genes cause the plant to produce its own Bt toxins. This so-called Bt concept has been used since 1996 and especially in corn and cotton.

Forestry

One of the most successful applications of Bt toxins is the fight against forest pests of the order butterflies, which cause great damage, especially in North American conifer forests. The use of Bt toxins led to a significant reduction of the use of chemical insecticides in forestry.

Combat mosquitoes

Bt toxins of B. thuringiensis subspecies israelensis are for different species of the genera Aedes, Culex and Anopheles, the disease carriers are toxic. In particular, can thus populations of vectors of diseases such as dengue fever, malaria, onchocerciasis and decimate. Since 1983 a program for the control of onchocerciasis is implemented in West Africa, which is based 80% on the use of Bt toxins. Thus an estimated 15 million children were protected from the disease. In mosquitoes so far no resistance development was documented.

Relationship with Bacillus cereus

In 2000, scientists set out in the journal Applied and Environmental Microbiology, that it is poisonous when alternative plant protection bacterium Bacillus thuringiensis, anthrax Bacillus anthracis and the Bacillus cereus food is the variations of a single species. The germs of Bacillus cereus form a hemolytic and a nichthämolytisches enterotoxin and cytotoxin K, ( a enterotoxisches protein). This bacterial toxin can cause human diarrhea not only significant, but also lead to death.

2001 was therefore Bacillus cereus IP 5832 banned as a probiotic in animal nutrition, because in the opinion of the Scientific Committee for feed the EU - Commission ( SCAN / Scientific Committee of Animal Nutrition ) by the bacterial strain pose a risk for the consumer.