Bacillus Thuringiensis

What Is Bacillus thuringiensis?

The bacteria Bacillus thuringiensis is an aerobic, Gram +ve and spore form. It is commonly known as Bt. It is found in soil, water, insect faeces, insects, plants surfaces, etc. It is widely used as a bioinsecticide to control the infestation of butterfly caterpillars. Caterpillars eat the spores sprayed onto plants and get killed.

Bacillus thuringiensis (Bt) produces an insecticidal protein during sporulation. The Bt-toxin gene is introduced into plants to develop a variety of pest-resistant plants by genetic engineering. E.g. Bt cotton, Bt corn, etc.

The activated Bt-toxin protein produced by Bacillus thuringiensis binds to the receptors in the epithelial cells of the midgut of insect larvae and causes swelling and lysis of the cell.

Classification and Characteristics

Bacillus thuringiensis (Bt) was discovered by Ishiwatari Shigetane in the silkworms. He named it Bacillus sotto. There are many known subspecies of Bacillus Thuringiensis, e.g. kurstaki (Btk), israelensis (Bti), etc.

• It is a rod-shaped bacterium (bacilli), about 2-5 µm long and 1.0 µm wide.
• It is a Gram +ve aerobic bacterium.
• It forms spores.
• The lifecycle of Bacillus thuringiensis comprises a vegetative stage and sporulation phase.
• Under favourable conditions, the vegetative cell divides by binary fission and reproduces two daughter cells. Spores also germinate to produce a vegetative cell under favourable conditions.
• When sufficient nutrients are not available or under adverse environmental conditions, the bacterium sporulates and forms spores and parasporal crystals.
• Bacillus thuringiensis produces endotoxin or delta endotoxin at the sporulation stage. It is a crystalline protein (cry proteins) and has insecticidal properties.
• It also produces cytolytic or Cyt proteins. Cyt proteins are mosquitocidal and are moderately toxic to some mosquito larvae.
• These proteins are encoded by cry genes. They are produced as protoxins that need to be activated in order to have a toxic effect.
• Most Cry proteins are effective against lepidopteran insects. Some are also effective against dipteran (flies), coleopteran (beetles) and also against nematodes.
• In most strains of Bacillus thuringiensis, cry genes are present in the plasmid.

Mechanism of Action of Bt-toxin

Bacillus thuringiensis is used to kill certain insects such as lepidopterans (tobacco budworm), coleopterans (beetles) and also some flies and mosquitoes (dipterans).

The Bt-toxin crystal protein is present as inactive protoxins in the bacteria. The protoxin gets activated inside the midgut of insect larvae. The alkaline pH present in the midgut solubilises the crystal and makes it active. The midgut proteases cleave the cry proteins after solubilisation at both C-terminal and N-terminal to make them active.

The active Bt-toxin binds to the glycoprotein or glycolipid receptors present on the epithelial cells of the midgut. It creates pores and forms a cation-selective channel in the cells and destroys the epithelial cells. As a result, the alkaline midgut juices flow into the haemolymph and increase the blood pH, leading to paralysis, inhibition of feeding and eventually to death.

If the receptors for Cry-proteins are absent, then the activated Cry-proteins are ineffective and are not toxic. e.g. in many lepidopterans and chewing insects.

Sucking insects, spiders and mites are not susceptible to the cry protein, because the ingestion of the endotoxin crystal is necessary for the toxic effect. Moreover, invertebrates that have a neutral or slightly acidic or basic environment in the midgut, remain unaffected by the Bt-toxin as alkaline pH is required for proteolysis of protoxin and make it active by dissolving the crystals. Under the highly acidic pH, as present in the stomach of humans and other vertebrates, the crystals of Cry proteins and Cyt proteins are solubilised but later degraded by proteolytic enzymes present in the gastric juices. They convert it to non-toxic peptides within minutes.

Bacillus thuringiensis Uses

Bacillus thuringiensis is widely used as a biocontrol agent and in genetic engineering to produce pest-resistant plants.

Biopesticide

Bacillus thuringiensis is an example of a biocontrol agent. It is used to kill the caterpillars of butterflies.

The first commercial bioinsecticide based on Bacillus thuringiensis, “Sporine”, was produced in France. It was used primarily to control flour moths.

Bacillus thuringiensis is sold as dried spores in the market for biocontrol. Vulnerable plants are sprayed by the spore mixture. When the insect larva eats the spores, they reach the gut. In the gut, the Bt-toxin gets activated leading to the death of the larvae. It kills only the caterpillars of the target insects and other insects and invertebrates are unharmed.

This bioinsecticide is used in fruit trees and flowering plants such as the genus Brassica.

Pest-resistant Plant

The Bt-toxin gene is cloned and inserted into the plant genome by recombinant DNA technology. These genetically modified (GM) plants express the Bt-toxin genes and become pest-resistant. These GM plants do not require the application of chemical insecticides. Some of the examples of crops are Bt cotton, Bt corn, soybean, potato, etc.

The Bt-toxin is coded by Cry genes. There are several Cry proteins coded by different Cry genes. The Bt-toxin is specific to insects. The gene is chosen according to the crop and the target pest. E.g. cryIAc and cryIIAb genes are effective against the cotton bollworms and cryIAb controls corn borer.

This was all about the Bacillus Thuringiensis. Learn more about other related concepts for NEET, only at BYJU’S.

Frequently Asked Questions

What is the use of Bacillus thuringiensis?

Bacillus thuringiensis (Bt) is regarded as the most effective microbial insecticide against several orders of insect pests. Additionally, Bt toxin genes have been successfully employed to increase insect pest resistance in genetically modified crops.

What is Bt toxin?

Bacillus thuringiensis (Bt) toxins are parasporal crystalline inclusions that contain proteins, some of which are deadly to a variety of insect species, nematodes, and human cancer cells. Thus the toxin produced by Bacillus thuringiensis (Bt) has been used as an insecticide spray.

Also check:

• MCQs On Biofertilizers
• MCQs On Pest Control
• Genetically Engineered Insulin
• Important Notes Of Biology For NEET Microbes In Human Welfare
• Flashcards Of Biology For NEET Microbes In Human Welfare
• Flashcards Of Biology For NEET Biotechnology Principles And Processes
• Important Notes Of Biology For NEET Application Of Biotechnology